refactor(app): extract view-model builders into scripts/views.py

Pull 350+ lines of inline per-row computation out of adults_view,
juniors_view, and payments into three pure builder functions with no
Flask globals or IO dependencies. Route handlers now contain only
cache/IO calls and a single render_template. No behaviour change —
all 27 tests pass.

Also moves get_month_labels, group_payments_by_person, and
adapt_junior_members out of app.py. Prep for /api/* shadow endpoints
(M5 Go parity).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/plans/2026-05-07-1431-m5-json-api-parity.md

@@ -0,0 +1,185 @@
+# Python view-model cleanup (M5 prep — Python side only)
+
+Scoped-down precursor to M5 of the Go rewrite. See:
+- [2026-05-03-2349-go-backend-rewrite.md](2026-05-03-2349-go-backend-rewrite.md)
+- [2026-05-03-2349-go-backend-rewrite-progress.md](2026-05-03-2349-go-backend-rewrite-progress.md)
+
+## Context
+
+The Python app is still production. Before adding `/api/X` shadow
+routes, JSON DTOs, parity tooling, and Go handlers, get the Python
+side into a clean shape. Today the `/adults` and `/juniors` routes
+each carry 150–200 lines of inline view-model construction inside
+the Flask handler:
+
+- [adults_view](app.py#L179) — 167 LOC, computes per-row
+  status/cell_text/balance/credits/debts inline.
+- [juniors_view](app.py#L347) — 205 LOC, same plus `"?"` sentinel
+  branching and `:NJ,MA` breakdown.
+- [payments](app.py#L553) — 35 LOC, lighter but still mixes IO and
+  grouping.
+
+Pulling that into pure builder functions:
+- shrinks `app.py` and makes each route handler do only what a route
+  handler should (IO + cache + step timing + render);
+- gives us **already-tested** pure functions that future M5 work can
+  call from a `/api/X` shadow endpoint with one line of `jsonify(...)`;
+- has zero behavioural change (existing tests in
+  [test_app.py](tests/test_app.py) act as the regression guard).
+
+This is a Python-only change. No Go work, no JSON contract, no shadow
+routes — those come after.
+
+## Approach
+
+Three pure builder functions in a new `scripts/views.py` module. Each
+takes already-loaded, deserialized inputs (no Flask globals, no IO,
+no cache calls) and returns the exact dict that today's route passes
+to `render_template`.
+
+```python
+def build_adults_view_model(
+    members, sorted_months, transactions, exceptions, current_month,
+    *, attendance_url, payments_url, bank_account,
+) -> dict: ...
+
+def build_juniors_view_model(
+    junior_members, sorted_months, transactions, exceptions, current_month,
+    *, attendance_url, payments_url, bank_account,
+) -> dict: ...
+
+def build_payments_view_model(
+    transactions, member_names,
+    *, attendance_url, payments_url,
+) -> dict: ...
+```
+
+The route handlers shrink to: load data (with `get_cached_data` and
+`record_step` calls staying in `app.py`), call the builder, render.
+
+```python
+@app.route("/adults")
+def adults_view():
+    members, sorted_months = ... # cached loads + record_step calls
+    transactions = ...
+    exceptions = ...
+    result_meta = ...
+    record_step("process_data")
+    vm = build_adults_view_model(
+        members, sorted_months, transactions, exceptions,
+        datetime.now().strftime("%Y-%m"),
+        attendance_url=..., payments_url=..., bank_account=BANK_ACCOUNT,
+    )
+    return render_template("adults.html", **vm)
+```
+
+## Decisions
+
+1. **Pure builders, no Flask state.** No `record_step`, no `g.*`, no
+   `get_cached_data` inside builders. They take plain args, return
+   plain dicts. This is what makes them trivially unit-testable and,
+   later, trivially reusable from `/api/X`.
+2. **Preserve byte-equal behaviour.** The dicts returned must match
+   today's `render_template(...)` kwargs key-for-key, value-for-value
+   — including the existing `json.dumps(member_data)` /
+   `json.dumps(month_labels_json)` / `json.dumps(raw_payments_json)`
+   wrappers. Those wrappers exist for inline JS in templates; the
+   refactor doesn't touch them. (When `/api/X` lands later, that route
+   will produce a sibling dict with the wrappers stripped, but that's
+   future work.)
+3. **`reconcile()` stays inside the route, not the builder.** It
+   crosses the IO/cache boundary in spirit (its inputs come from
+   cache); but it's also pure-domain and called by both adults and
+   juniors. Keep the call site in the route so `record_step("reconcile")`
+   timing isn't lost. Builder takes `result` as an argument.
+4. **Shared helpers stay in `app.py`** for now —
+   [get_month_labels](app.py#L41) and
+   [group_payments_by_person](app.py#L60) are already module-level
+   pure functions, used by routes and now by builders. Either leave
+   them in `app.py` and import into `scripts/views.py`, or move them
+   into `scripts/views.py`. **Choose: move into `scripts/views.py`**
+   — they're view-model concerns, not Flask concerns, and `app.py`
+   should keep shrinking.
+5. **No new test file needed.** Existing
+   [tests/test_app.py](tests/test_app.py) tests
+   `test_adults_route`, `test_juniors_route`, `test_payments_route`
+   exercise the rendered HTML end-to-end. If they pass after the
+   refactor, behaviour is preserved. Adding builder-level unit tests
+   is a *nice-to-have* but not required for this iteration.
+
+## Tasks
+
+### 1. Create `scripts/views.py` with the three builders + shared helpers
+
+- Move `get_month_labels` and `group_payments_by_person` from
+  [app.py:41–77](app.py#L41-L77) into `scripts/views.py`. Update the
+  import in `app.py`.
+- Implement `build_adults_view_model` by extracting
+  [app.py:200–344](app.py#L200-L344) (everything between `result =
+  reconcile(...)` and `return render_template(...)`). Take `result`
+  as a parameter; emit the same dict that's currently passed as
+  `**kwargs` to `render_template`.
+- Implement `build_juniors_view_model` by extracting
+  [app.py:370–550](app.py#L370-L550). Same shape — including the
+  `adapted_members` adapter loop, the junior `?`-sentinel branches,
+  and the `:NJ,MA` breakdown.
+- Implement `build_payments_view_model` by extracting
+  [app.py:570–586](app.py#L570-L586) (the `group_payments_by_person`
+  call + `Unmatched / Unknown` bucket + sort).
+
+### 2. Slim down the route handlers in `app.py`
+
+Each handler keeps:
+- `attendance_url`/`payments_url` URL building
+- `get_cached_data` calls (with `record_step` between)
+- `reconcile(...)` call (adults/juniors) with `record_step("reconcile")`
+- `record_step("process_data")` after the builder call
+- `return render_template("X.html", **view_model)`
+
+Each handler **drops** all the per-row computation, totals
+formatting, credits/debts sorting, `raw_payments_by_person` building,
+and `import json` lines.
+
+Target post-refactor LOC for each route handler: ~25–30 lines
+(currently 167 / 205 / 35).
+
+### 3. Run the existing test suite + manual smoke
+
+```
+make test                # all tests green
+make web-py              # browse /adults /juniors /payments visually
+```
+
+## Critical files
+
+- [app.py](app.py) — routes shrink dramatically; `get_month_labels`
+  and `group_payments_by_person` get moved out.
+- New: `scripts/views.py` — three builders + the two helpers.
+- [tests/test_app.py](tests/test_app.py) — unchanged; serves as the
+  regression guard.
+
+## Verification
+
+1. `make test` — all four `TestWebApp` tests pass unchanged.
+2. `make web-py` and visit `/adults`, `/juniors`, `/payments` — each
+   renders identically to before (same table contents, same totals,
+   same credits/debts, same `?` rendering on juniors).
+3. `git diff app.py` shows substantial deletions (route bodies
+   shrink) and only thin glue calling the new builders.
+4. Optional sanity check: temporarily add `print(repr(view_model))`
+   in the route before `render_template` on `main` and on the branch,
+   diff for one fixture run — should be byte-identical dicts.
+
+## Out of scope (future iterations, not this plan)
+
+- `/api/<route>` shadow endpoints in Flask
+- Go `internal/web/api/` DTOs and JSON Schemas
+- Go `internal/services/membership/views.go` aggregators
+- Go HTTP handlers for `/api/X`
+- `cmd/parity/main.go` and `make parity` target
+- Junior breakdown sidecar work in
+  `go/internal/services/membership/sources.go`
+
+These are the original M5 tasks (M5.1–M5.4 in the progress tracker).
+They become much easier once today's refactor lands, because the
+shadow `/api/X` routes will be one-liners over the new builders.