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>

go/tests/fixtures/pure/infer_transaction_details/other_prefix.json

@@ -0,0 +1,36 @@
+{
+  "case": "other_prefix",
+  "func": "scripts.match_payments.infer_transaction_details",
+  "captured_at": "2026-10-08",
+  "input": {
+    "tx": {
+      "sender": "Jana Novakova",
+      "message": "other: tournament leden 2026",
+      "user_id": "",
+      "date": {
+        "type": "string",
+        "value": "2026-04-15"
+      }
+    },
+    "member_names": [
+      "Alžběta Testovická",
+      "Tomáš Fiktivný (Tov)",
+      "Pavel Smutný (Štrúdl)",
+      "Jana Nováková",
+      "Adam Novák"
+    ],
+    "default_year": 2026
+  },
+  "output": {
+    "matches": [
+      {
+        "name": "Jana Nováková",
+        "confidence": "auto"
+      }
+    ],
+    "months": [
+      "other: tournament leden 2026"
+    ],
+    "search_text": "Jana Novakova other: tournament leden 2026 "
+  }
+}