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>

app/db/models.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timezone
 
-from sqlalchemy import BigInteger, ForeignKey, Index, String, UniqueConstraint, text
+from sqlalchemy import BigInteger, ForeignKey, Index, String, Text, UniqueConstraint, text
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.db.base import Base
@@ -154,7 +154,7 @@ class PendingTransaction(Base):
     user_id: Mapped[int | None] = mapped_column(ForeignKey("users.id"), default=None)
     current_txid: Mapped[str] = mapped_column(String(64))
     fee_rate_sat_vb: Mapped[int]
-    raw_tx_hex: Mapped[str] = mapped_column(String)
+    raw_tx_hex: Mapped[str] = mapped_column(Text)
     # The *first* broadcast — never rewritten by a bump — since this is what the
     # reconciler's abandon-after-N-hours grace period (app/tx/reconcile.py) measures
     # from. Bumping used to overwrite this field, which reset that clock on every
@@ -193,7 +193,7 @@ class AuditLog(Base):
 
     id: Mapped[int] = mapped_column(primary_key=True)
     event_type: Mapped[str] = mapped_column(String(32))
-    payload_json: Mapped[str] = mapped_column(String)
+    payload_json: Mapped[str] = mapped_column(Text)
     user_id: Mapped[int | None] = mapped_column(ForeignKey("users.id"), default=None)
     round_id: Mapped[int | None] = mapped_column(ForeignKey("rounds.id"), default=None)
     created_at: Mapped[datetime] = mapped_column(default=utcnow)