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.
