fix: Distribute multi-month payments by per-month expected fee

reconcile() previously split a multi-month payment evenly across months,
which falsely flagged months as underpaid when their expected fees
differed (e.g. 1250 CZK for 02+03+04 2026 with rates 750/350/150 was
shown as 416/month with two months red).

The allocation now runs per matched member: greedy when the share covers
the total expected (each month gets its expected fee, surplus -> credit),
proportional by expected fee otherwise. Out-of-window months keep the
previous even-split-to-credit behavior. 6 new test cases.

Also adds CHANGELOG.md and a changelog convention in CLAUDE.md.

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

CLAUDE.md

@@ -16,22 +16,95 @@ Flask-based financial management system for FUJ (Frisbee Ultimate Jablonec). Han
 This project uses `uv` for dependency management.
 
 ```bash
-uv venv          # Create virtual environment
-uv sync          # Install dependencies from pyproject.toml
+uv venv && uv sync
 source .venv/bin/activate
 ```
 
-Alternatively, use the Makefile:
-- `make sync`      - Sync bank transactions to Google Sheets
-- `make infer`     - Automatically infer Person/Purpose/Amount in the sheet
-- `make reconcile` - Generate balance report from Google Sheets data
-- `make fees`      - Calculate expected fees from attendance
-- `make match`     - (Legacy) Match bank data directly
-- `make web`       - Start dashboard
-- `make image`     - Build Docker image
+Set `PYTHONPATH=scripts:.` when running scripts directly (the Makefile does this automatically).
 
-Requires `.secret/fuj-management-bot-credentials.json` for Google Sheets API access (configurable via `CREDENTIALS_PATH` env var).
+## Commands
+
+```bash
+make web          # Start dashboard at http://localhost:5001
+make web-debug    # Same with FLASK_DEBUG=1
+make test         # Run all tests (unittest discover)
+make test-v       # Tests with verbose output
+make fees         # Print fee report from attendance sheet
+make sync-2026    # Sync Fio bank transactions for 2026 to Google Sheets
+make infer        # Auto-fill Person/Purpose/Amount columns in payments sheet
+make reconcile    # Print balance report from Google Sheets data
+make image        # Build Docker image
+```
+
+Run a single test:
+```bash
+PYTHONPATH=scripts:. python -m unittest tests.test_app.TestWebApp.test_adults_route
+```
+
+## Architecture
+
+### Data flow
+
+```
+Google Sheets (attendance) ──► attendance.py ──► reconcile() ──► Flask routes ──► templates/
+Google Sheets (payments)   ──► match_payments.py ──┘
+Fio Bank API               ──► sync_fio_to_sheets.py ──► Google Sheets (payments)
+```
+
+### Key modules
+
+- `app.py` — Flask app; routes for `/adults`, `/juniors`, `/payments`, `/sync-bank`, `/qr`, `/flush-cache`
+- `scripts/attendance.py` — Fetches attendance CSV from Google Sheets, computes per-member per-month fees. Contains fee rate constants (`ADULT_FEE_DEFAULT`, `JUNIOR_FEE_DEFAULT`) and `ADULT_MERGED_MONTHS` / `JUNIOR_MERGED_MONTHS` dicts.
+- `scripts/match_payments.py` — `reconcile()` matches transactions to members/months. `fetch_sheet_data()` reads the payments sheet. `fetch_exceptions()` reads the `exceptions` tab.
+- `scripts/cache_utils.py` — Invalidation via Google Drive API `modifiedTime`; falls back to 5-minute TTL buckets when Drive API is unavailable. Cache files live in `tmp/`.
+- `scripts/sync_fio_to_sheets.py` — Pulls Fio bank transactions and appends them to the payments Google Sheet.
+- `scripts/infer_payments.py` — Fills in Person/Purpose/Inferred Amount columns using name-matching heuristics.
+- `scripts/config.py` — All external IDs, paths, and tunable TTLs. Override via env vars (`CREDENTIALS_PATH`, `BANK_ACCOUNT`, `CACHE_TTL_SECONDS`).
+
+### Member tiers
+
+Tiers are set in column B of the attendance sheet:
+- `A` — Adult, pays fees (750 CZK/month for 2+ sessions, 200 CZK for exactly 1)
+- `J` — Junior attending adult practices; their attendance is merged with the junior sheet
+- `X` — Excluded from junior fee calculation (coaches, etc.)
+
+### Fee calculation
+
+- Adults: 0 sessions → 0, 1 session → 200 CZK, 2+ sessions → monthly rate (default 750 CZK)
+- Juniors: 0 → 0, 1 → `"?"` (manual review required), 2+ → monthly rate (default 500 CZK)
+- Per-member per-month overrides live in the `exceptions` tab of the payments sheet (columns: Name, Period YYYY-MM, Amount, Note). Exceptions are keyed by `(normalize(name), normalize(period))`.
+
+### Merged months
+
+`ADULT_MERGED_MONTHS` / `JUNIOR_MERGED_MONTHS` in `attendance.py` map a source month to a target month (e.g., `"2025-12": "2026-01"` merges December into January billing). The target month accumulates attendance from both months.
+
+### Caching
+
+`get_cached_data()` in `app.py` checks the Drive API `modifiedTime` before each request and serves a JSON file from `tmp/` when the sheet hasn't changed. Cache is warmed up at startup (`warmup_cache()`). Flush via `/flush-cache` (POST) or `flush_cache()`.
+
+### Payments sheet columns
+
+`Date | Amount | manual fix | Person | Purpose | Inferred Amount | Sender | VS | Message | Bank ID | Sync ID`
+
+`Person` and `Purpose` are written by `infer_payments.py` and can be manually corrected. `manual fix` column presence disables re-inference for that row. Multiple people or months are comma-separated in Person/Purpose.
+
+### QR codes
+
+`/qr?account=…&amount=…&message=…` generates a Czech QR Platba PNG (SPD format).
 
 ## Git Commits
 
-When making git commits, always append yourself as co-author trailer to the end of the commit message to indicate AI assistance
+When making git commits, always append yourself as co-author trailer to the end of the commit message to indicate AI assistance
+
+## Changelog
+
+Maintain a running changelog in `CHANGELOG.md` at the repo root. After every significant change, fix, or update — once the user confirms it works — append a new entry **at the top** of the file in this format:
+
+```markdown
+## YYYY-MM-DD HH:MM TZ — short title
+
+- One-line summary of what changed and why.
+- Key files touched (optional, only if useful for traceability).
+```
+
+Get the timestamp with `date "+%Y-%m-%d %H:%M %Z"`. Skip trivial edits (typos, formatting, comment tweaks); only log changes a future reader would care about.