Widen raw_tx_hex and payload_json from String to Text (B-47)

Both held arbitrary-length data (a raw signed transaction hex, an audit
payload) in a bare String, which SQLAlchemy compiles to VARCHAR with no
length. SQLite and PostgreSQL accept that; other backends like MySQL
require a length on VARCHAR and would reject it. Add a migration
(verified upgrade/downgrade/upgrade round-trip, and confirmed with
`alembic check` that it leaves no further diff against the models).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/unit/test_models_text_columns.py

@@ -0,0 +1,16 @@
+"""B-47: raw_tx_hex (a full raw signed transaction hex) and payload_json (an
+arbitrary audit payload) must stay `Text`, not a bare `String`/`VARCHAR` with
+no length -- SQLite and PostgreSQL accept that, but other backends (e.g.
+MySQL) require a length on VARCHAR and would reject it."""
+
+from sqlalchemy import Text
+
+from app.db.models import AuditLog, PendingTransaction
+
+
+def test_pending_transaction_raw_tx_hex_is_text():
+    assert isinstance(PendingTransaction.__table__.c.raw_tx_hex.type, Text)
+
+
+def test_audit_log_payload_json_is_text():
+    assert isinstance(AuditLog.__table__.c.payload_json.type, Text)