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/reconcile/11_other_message_fallback.json

@@ -0,0 +1,69 @@
+{
+  "case": "11_other_message_fallback",
+  "func": "scripts.match_payments.reconcile",
+  "captured_at": "2026-10-08",
+  "input": {
+    "members": [
+      {
+        "name": "Member_d035d9f9",
+        "tier": "A",
+        "fees": {
+          "2026-01": [
+            750,
+            3
+          ]
+        }
+      }
+    ],
+    "sorted_months": [
+      "2026-01"
+    ],
+    "transactions": [
+      {
+        "date": "2026-01-10",
+        "amount": 300,
+        "manual_fix": "",
+        "person": "",
+        "purpose": "",
+        "inferred_amount": 300,
+        "sender": "Member_d035d9f9",
+        "message": "other: tournament",
+        "bank_id": ""
+      }
+    ],
+    "exceptions": [],
+    "default_year": 2026
+  },
+  "output": {
+    "members": {
+      "Member_d035d9f9": {
+        "tier": "A",
+        "months": {
+          "2026-01": {
+            "expected": 750,
+            "original_expected": 750,
+            "attendance_count": 3,
+            "exception": null,
+            "paid": 0.0,
+            "transactions": []
+          }
+        },
+        "other_transactions": [
+          {
+            "amount": 300.0,
+            "date": "2026-01-10",
+            "sender": "Member_d035d9f9",
+            "message": "other: tournament",
+            "purpose": "other: tournament",
+            "confidence": "auto"
+          }
+        ],
+        "total_balance": -750
+      }
+    },
+    "unmatched": [],
+    "credits": {
+      "Member_d035d9f9": -750
+    }
+  }
+}