# Fix the two Go/Python parity differences

## Context

Running `make parity` against both containers on 2026-10-08 reported
`0/3 routes match`. Two causes, neither related to the move to `legacy/`:

1. **VS rendered in scientific notation by Go.** The payments sheet is read
   with `UNFORMATTED_VALUE`, so a numeric VS cell arrives as `float64`. Go
   stringifies cells with `fmt.Sprint`, which prints any float ≥ 1e6 in
   exponent form (`2.609767878e+09`). Python already converts integer floats
   to plain digits for VS (`get_str` in `fetch_sheet_data`). VS must always be
   a plain string.
2. **`months_to_show` missing from the Python JSON API.** Go returns it in
   `/api/adults` and `/api/juniors` (and it is `required` in the committed
   schemas). Python already has the feature in its HTML pages
   (`MONTHS_TO_SHOW` is passed to the templates) but never added the key to
   the API responses. Backport the key so the API contract matches.

Outcome: `make parity` reports 3/3 routes matching.

## Branching

Both are bug fixes that touch `legacy/` paths, so they depend on the
uncommitted `feat/python-to-legacy` work. Commit that branch first (still
awaiting confirmation), then branch `fix/api-parity-vs-months-to-show` from
it, with one commit per task. Open the MR with `tea` after the legacy move is
merged, or target it at that branch.

## Task 1 — VS (and other text cells) always a plain string in Go

Root cause is `getVal` in `parseTransactionRows`
(`go/internal/services/membership/sources.go`, the `fmt.Sprint(row[i])`
closure). The same pattern exists in `get` in
`go/internal/services/banksync/infer.go`.

1. Add `CellString(v any) string` to `go/internal/io/sheets` (new small file
   `cell.go`; both services already import this package):
   - `nil` → `""`
   - `float64` → `strconv.FormatFloat(x, 'f', -1, 64)` (never exponent form;
     `2609767878.0` → `"2609767878"`, `12.5` → `"12.5"`)
   - `string` → unchanged
   - anything else → `fmt.Sprint(v)`
2. Use it in `getVal` in `sources.go`. This fixes VS and protects the other
   text columns (Message, Bank ID, Sender, …) from the same failure.
   - `getVal` currently returns `"<nil>"` for nil cells and the inferred-amount
     code checks for that literal; simplify that check to `iv != ""`.
   - Leave `getRaw`/`FormatDate` and `parseFloat` for Date and Amount as they
     are — those must stay numeric.
3. Use it in `get` in `banksync/infer.go` (keeping the `TrimSpace`), so
   inference sees the same text the dashboard shows.
4. Tests:
   - `go/internal/io/sheets/cell_test.go`: table test for `CellString`
     (large integer float, small integer float, fractional, string with
     leading zeros, nil).
   - Extend `TestLoadTransactions` in
     `go/internal/services/membership/sources_test.go` with a row whose VS is
     `2609767878.0` and whose Bank ID is an 11-digit float; assert plain
     digit strings.
5. Python: no code change needed — `get_str` already does this for VS. Add one
   regression test for `fetch_sheet_data` in
   `legacy/tests/test_match_payments.py` (mock the Sheets service, numeric VS
   cell → `"2609767878"`) so the two sides are pinned to the same behaviour.

Not included (note for later): VS is *written* to the sheet with
`USER_ENTERED` by both sync implementations, so Sheets stores it as a number
and drops leading zeros. Fixing that means writing VS as text and is a
separate change to the sheet's data.

## Task 2 — backport `months_to_show` to the Python JSON API

`legacy/app.py` only; `MONTHS_TO_SHOW` is already imported there.

1. In `api_adults()` and `api_juniors()`, add the key to the response:
   build `out = _unwrap_view_model_for_api(vm)`, set
   `out["months_to_show"] = MONTHS_TO_SHOW`, return `jsonify(out)`.
   - Do not add it inside `build_*_view_model` or `_unwrap_view_model_for_api`:
     the HTML routes already pass `months_to_show=` to `render_template`
     alongside `**vm`, and a duplicate key would raise `TypeError`.
   - `/api/payments` is unchanged; Go does not return the key there.
2. Tests in `legacy/tests/test_app.py`:
   - Add `"months_to_show"` to the expected key list used by
     `test_api_adults` / `test_api_juniors` (the tuple near the top of the
     file), and assert the value is an `int`.
3. Go: no change. The key and schemas
   (`go/tests/fixtures/api-schema/{adults,juniors}.schema.json`) already
   contain it.

## Wrap-up

- `CHANGELOG.md` entry at the top (after confirmation it works), per repo
  convention.
- Copy this plan to
  `docs/plans/2026-10-08-HHMM-api-parity-vs-months-to-show.md`, per the repo's
  plan-location rule.
- Add both items to
  `docs/plans/2026-10-08-1026-pre-feature-cleanup-tasks.md` as done tasks so
  the audit list stays complete.

## Verification

1. `make go-test-all` — unit tests (including the new ones) and parity
   fixtures pass.
2. `make test` — Python tests pass, including the new API key and VS tests.
3. `cd go && go test ./internal/web/api/...` — schema golden test still
   matches (no schema change expected).
4. End to end: start `make web-py` and `make web-go`, then `make parity`;
   expect `3/3 routes match`. Alternatively repeat the two-container run used
   to find the bugs (`make image`, Go image from `go/build/Dockerfile`, then
   `go run ./cmd/parity -py … -go …`).
5. Open `/payments` on :8080 and check the affected transaction's VS shows as
   plain digits.