feat: merge a link's extra into the payout payment (v1.2.2-aio.2)

Adds an optional `extra` (JSON) field to a withdraw link. When the link
is claimed, that `extra` is merged onto the payout payment's `extra`, so
a caller can tag the resulting payment with metadata an external listener
keys on — the link is the only place to attach it (the customer-facing
LNURL-withdraw payout otherwise carries just `{tag, withdrawal_link_id}`).

Motivating use: bitSpire cash-in settlements. The operator's spirekeeper
listener fires a `cash_in` settlement (fee split to the platform) only on
an outbound payment stamped `source=bitspire`; before this there was no
way to stamp an LNURL-withdraw payout, so cash-ins never settled. bitSpire
now creates the cash-in link for the NET amount with
`extra={source, type:cash_in, principal_sats, fee_sats, ...}` and the
settlement fires on claim.

- models: `extra: dict | None` on CreateWithdrawData + WithdrawLink.
  LNbits' db layer (de)serializes dict columns to/from JSON natively
  (same as Payment.extra) — no per-field validator needed.
- migrations_fork.py: `withdraw_link.extra TEXT` under `withdraw_fork`,
  keeping the upstream-tracked migrations.py byte-identical for clean
  rebases (aiolabs/lnbits#8 pattern).
- views_lnurl: `extra={**(link.extra or {}), "tag": ..., "withdrawal_link_id": ...}`
  — the withdraw extension's own keys are written last so a caller cannot
  clobber them.

Verified end-to-end on the dev stack: a stamped link's payout carries the
merged extra and drives a spirekeeper cash_in settlement + super-fee payout.

migrations_fork.py

@@ -0,0 +1,44 @@
+"""
+Fork-specific database migrations for the aiolabs withdraw extension.
+
+These migrations are tracked separately under `withdraw_fork` in the
+`dbversions` table (loaded by `lnbits/core/helpers.py:migrate_extension_database`),
+so they do not collide with upstream's `m{NNN}_*` numbering in
+`migrations.py`. Keeping the upstream-tracked file untouched means
+`git pull upstream` stays rebase-clean for schema changes.
+
+Conventions:
+  - Sequential numbering starting from m001.
+  - Each migration is `async def m{NNN}_<description>(db)`.
+  - DDL must be idempotent: a fresh install runs every migration; an
+    install that already carries the column must not crash. Use
+    `_alter_add_column_safe` so re-runs are no-ops.
+"""
+
+
+async def _alter_add_column_safe(db, sql: str) -> None:
+    """ALTER TABLE ADD COLUMN that swallows duplicate-column errors, so a
+    re-run on a DB that already has the column is a silent no-op."""
+    try:
+        await db.execute(sql)
+    except Exception as exc:
+        msg = str(exc).lower()
+        if "duplicate column" in msg or "already exists" in msg:
+            return
+        raise
+
+
+async def m001_aio_withdraw_schema(db):
+    """
+    Apply every aiolabs schema delta on top of upstream withdraw.
+
+    `withdraw_link.extra` — arbitrary JSON merged into the payout payment's
+    `extra` when the link is claimed (see views_lnurl). Lets a caller tag the
+    resulting payment with settlement/attribution metadata an external listener
+    can key on — e.g. bitSpire stamps {source, type, principal_sats, fee_sats,
+    ...} so the spirekeeper cash-in settlement fires off an LNURL-withdraw
+    payout. Stored as TEXT; (de)serialized to a dict by the WithdrawLink model.
+    """
+    await _alter_add_column_safe(
+        db, "ALTER TABLE withdraw.withdraw_link ADD COLUMN extra TEXT"
+    )