feat(infer): messages starting with "other:" are not monthly fees

When the bank message starts with "other:" (case-insensitive),
inference does not look for a month, neither in the text nor from the
payment date. The trimmed message is written to Purpose, so reconcile
books the payment as an "other" transaction for the person instead of
a monthly fee. Messages containing "other:" elsewhere are unchanged.

- matching.IsOtherPurpose / is_other_purpose shared by inference and
  reconcile
- reconcile applies the rule to rows not yet processed by inference
- no person found: Purpose is still written, row shows as unmatched
- implemented in Go and legacy Python; four new parity fixtures

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

legacy/scripts/_fixture_seeds.py

@@ -358,6 +358,36 @@ SEEDS[("infer_transaction_details", "serial_date")] = {
     "member_names": _ROSTER,
     "default_year": 2026,
 }
+SEEDS[("infer_transaction_details", "other_prefix")] = {
+    "tx": {
+        "sender": "Jana Novakova",
+        "message": "other: tournament leden 2026",
+        "user_id": "",
+        "date": {"type": "string", "value": "2026-04-15"},
+    },
+    "member_names": _ROSTER,
+    "default_year": 2026,
+}
+SEEDS[("infer_transaction_details", "other_prefix_uppercase")] = {
+    "tx": {
+        "sender": "Test Payer",
+        "message": " Other: Dres ",
+        "user_id": "",
+        "date": {"type": "string", "value": "2026-04-15"},
+    },
+    "member_names": _ROSTER,
+    "default_year": 2026,
+}
+SEEDS[("infer_transaction_details", "other_not_at_start")] = {
+    "tx": {
+        "sender": "Jana Novakova",
+        "message": "leden other: dres",
+        "user_id": "",
+        "date": {"type": "string", "value": "2026-04-15"},
+    },
+    "member_names": _ROSTER,
+    "default_year": 2026,
+}
 SEEDS[("infer_transaction_details", "no_member_no_month")] = {
     "tx": {
         "sender": "Unknown Person",
@@ -476,6 +506,14 @@ SEEDS[("reconcile", "07_other_purpose_split")] = _reconcile_seed(
     transactions=[_tx("2026-01-10", 800, "Alice Dvořák, Bob Kratochvíl", "other:tournament", sender="Alice Dvořák")],
 )
 
+# 11 — message starts with "other:" on a row not yet inferred (empty Person/Purpose):
+# goes to the other ledger, the month stays unpaid
+SEEDS[("reconcile", "11_other_message_fallback")] = _reconcile_seed(
+    members=[_member("Alice Dvořák", "A", {"2026-01": (750, 3)})],
+    sorted_months=["2026-01"],
+    transactions=[_tx("2026-01-10", 300, "", "", sender="Alice Dvořák", message="other: tournament")],
+)
+
 # 08 — junior with attendance=1 (expected=0 in reconcile, unknown in UI)
 SEEDS[("reconcile", "08_junior_question_mark")] = _reconcile_seed(
     members=[_member("Karel Junior", "A", {"2026-01": (0, 1)})],