# Inference: messages starting with "other:" are not monthly fees

## Context

Some payments are not monthly fees (tournament fees, jerseys, …). Reconcile
already supports them: a row whose **Purpose** starts with `other:` is attached
to the person's "other transactions" and is not counted against any month.
Today that only happens when someone types the purpose by hand, because
inference always tries to find a month — from the message, or failing that
from the payment date.

New rule: when the bank message for the recipient **starts with `other:`**,
inference must not look for a month. It copies the message into Purpose, so
the payment is linked to the person but not counted as a monthly fee. Messages
that merely contain `other:` somewhere else are handled the old way.

Implemented in both Go and legacy Python (this is inference behaviour, so the
dual implementation rule applies).

## Behaviour

- Trigger: message, after trimming whitespace, starts with `other:`
  (case-insensitive, same test reconcile already uses for Purpose).
- Purpose written to the sheet: the trimmed message, unchanged
  (e.g. `other: turnaj Praha`). No month parsing, no date fallback.
- Person: inferred exactly as today (sender + message + VS, then sender
  alone), including the `[?]` review marker.
- Inferred Amount: the full amount, as today.
- No person found: Purpose is still written, Person stays empty. The row then
  shows up under "unmatched" for a manual fix instead of being guessed again
  on every run.
- Rows not yet processed by inference (Person and Purpose both empty) are
  inferred on the fly by reconcile; that path follows the same rule, so such a
  payment is never briefly counted as a monthly fee.
- Existing rows are untouched: inference only fills rows where manual fix,
  Person and Purpose are all empty. To reprocess an old row, clear its Person
  and Purpose cells.

## Implementation

Branch `feat/infer-other-purpose` off `main`; MR via `tea`.

The output shape of the inference function does not change: for an "other"
message the months list holds the single purpose string. That is the same
convention reconcile already uses internally (`matchedMonths = [purpose]`),
and it means the sheet-writing code needs no change in either language.

1. **Shared predicate**
   - Go: `matching.IsOtherPurpose(s string) bool` in
     `go/internal/domain/matching/infer.go`
     (`strings.HasPrefix(strings.ToLower(strings.TrimSpace(s)), "other:")`).
   - Python: `is_other_purpose(s)` in `legacy/scripts/match_payments.py`.
   - Replace the two existing inline checks in reconcile with it
     (`go/internal/domain/reconcile/reconcile.go`, `reconcile()` in
     `match_payments.py`).

2. **Inference**
   - Go `matching.InferTransactionDetails` and Python
     `infer_transaction_details`: after member matching, if the message is an
     other-purpose, return `months = [trimmed message]` and skip
     `ParseMonthReferences` and the date fallback. Update the doc comments
     (months may hold one `other:…` purpose).
   - `banksync.InferPayments` (`go/internal/services/banksync/infer.go`) and
     `infer_payments.py`: no logic change — they already join months into
     Purpose and write the row when either members or months are non-empty.

3. **Reconcile fallback** (rows with empty Person/Purpose)
   - In the inference-fallback branch of both reconcile implementations: if
     the inferred months are a single other-purpose, set the row's purpose to
     it and mark it as other, so it goes to the member's other-transactions
     ledger instead of the month allocation.

4. **Tests and fixtures**
   - Seeds in `legacy/scripts/_fixture_seeds.py`, captured with the existing
     capture/scrub scripts for these two functions only:
     - `infer_transaction_details`: `other_prefix` (message
       `other: tournament`, member in sender → that purpose, no month),
       `other_prefix_uppercase` (`Other: …`), `other_not_at_start`
       (`leden other: x` → old behaviour, month parsed).
     - `reconcile`: `11_other_message_fallback` (empty Person/Purpose, message
       starts with `other:` → lands in `other_transactions`, month stays
       unpaid).
   - Go unit tests: `IsOtherPurpose` table; cases in
     `matching/infer_test.go`; a case in `banksync/infer_test.go` asserting
     the written row (`Person`, `Purpose = "other: …"`, amount) and one with
     no matching member; a fallback case in `reconcile/reconcile_test.go`.
   - Python: cases in `legacy/tests/test_match_payments.py` for
     `infer_transaction_details` and the reconcile fallback.
   - Existing fixtures must stay byte-identical apart from the new files.

5. **Docs**
   - `CLAUDE.md`: add the rule to the payments-sheet / inference description.
   - `CHANGELOG.md` entry; copy this plan to
     `docs/plans/2026-10-08-HHMM-infer-other-purpose.md`.

No change to the JSON API, schemas, templates or the sheet layout.

## Verification

1. `make go-test-all` (unit + parity, including the new fixtures) and
   `make test`.
2. `git status go/tests/fixtures` shows only the four new fixture files.
3. Dry run against the live sheet in both implementations
   (`fuj infer --dry-run`, Python `infer_payments.py --dry-run`): confirm it
   runs cleanly and proposes nothing unexpected. The live sheet probably has
   no un-inferred row starting with `other:`, so the new rule itself is
   proven by the tests rather than by live data.
4. `make parity` between the two servers (or the two-container run used
   earlier) still reports 3/3 routes matching.