feat: Grin slatepack relay (non-custodial async mailbox, feature-flagged)

Final wallet-handler slice. An async store-and-forward mailbox for interactive
Grin transfers between two registered Smirk users, over the grin_slatepacks db.
Reworked clean from the legacy relay — NOT a port.

Endpoints (under /api/v1/wallet/grin/relay, JWT-gated):
- POST create   {recipient_user_id, slate_id, slatepack, amount_nanogrin}
- GET  pending  -> the caller's inbox
- POST get      {slate_id} -> poll a relay's state (sender/recipient)
- POST respond  {slate_id, response_slatepack} (recipient)
- POST finalize {slate_id, tx_hash} (sender; RECORDS the txid only)
- POST cancel   {slate_id} (sender or recipient)

Non-custodial: stores only opaque (encrypted) slatepacks + lifecycle status;
never holds keys, finalizes, or broadcasts. The wallet broadcasts locally (via
/wallet/grin/broadcast) and reports the txid here — diverging from the legacy
server-side post_tx (see docs/private/OWN_BACKEND_CONVERGENCE.md).

Controls + hardening (incl. fixes from the adversarial review workflow):
- Per-action authz: caller must be the slate's sender/recipient; a non-party
  gets 404 (no existence oracle).
- slate_id is UNIQUE (+ validated as a UUID) and mutations key on the
  authorized row's PRIMARY KEY, not slate_id — so a duplicate slate_id can't
  brick a relay (fetch_optional-on-2-rows 500) or let a slate_id-keyed UPDATE
  touch another row. create maps the unique violation to 409.
- State transitions are ATOMIC: the status + expiry preconditions live in the
  UPDATE WHERE clause (not a Rust read-check-write), so respond-twice and
  cancel-races-finalize cannot occur; a non-qualifying transition -> 409.
- TTL enforced at the action boundary (respond/finalize require expires_at >
  now) + a periodic expire_old_slatepacks sweep in main.rs (relay-enabled only).
- Recipient must be a registered user; slatepack sizes capped (64 KiB).
- Behind FEATURE_GRIN_RELAY (default on).

6 paths + schemas + grin_relay tag in openapi (drift-clean). 4 L1 integration
tests against live Postgres: full lifecycle, unregistered-recipient 400, auth
gate, non-party/non-recipient 404s.

migrations/20260622000001_initial_schema.sql

@@ -84,7 +84,10 @@ CREATE INDEX idx_user_keys_user_id ON user_keys (user_id);
 -- ── grin_slatepacks ─────────────────────────────────────────────────────────
 CREATE TABLE grin_slatepacks (
     id                UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-    slate_id          TEXT NOT NULL,
+    -- UNIQUE: the relay's authorization + state machine key off slate_id, so it
+    -- must map to exactly one row (a collision would let mutations keyed on
+    -- slate_id touch another user's relay). Also serves as the lookup index.
+    slate_id          TEXT NOT NULL UNIQUE,
     sender_user_id    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
     recipient_user_id UUID REFERENCES users(id) ON DELETE SET NULL,
     recipient_address TEXT,
@@ -98,7 +101,7 @@ CREATE TABLE grin_slatepacks (
     finalized_at      TIMESTAMPTZ,
     tx_hash           TEXT
 );
-CREATE INDEX idx_grin_slatepacks_slate_id ON grin_slatepacks (slate_id);
+-- (slate_id lookup index is provided by the UNIQUE constraint above.)
 CREATE INDEX idx_grin_slatepacks_sender ON grin_slatepacks (sender_user_id);
 CREATE INDEX idx_grin_slatepacks_recipient ON grin_slatepacks (recipient_user_id);
 CREATE INDEX idx_grin_slatepacks_status ON grin_slatepacks (status);