feat(registration): composable gate framework + invite codes (gate #1)

First of the operator-configurable registration gates (PoW-priced restore and
pay-to-register follow). A NEW wallet must present a valid invite code when
REGISTRATION_REQUIRE_INVITE is on; returning wallets and self-hosting bypass it,
and the active gates are advertised via /capabilities so the wallet shapes
onboarding.

- migration + db: invite_codes table storing only sha256(code) (128-bit codes,
  no pepper needed). Redemption is an atomic single-statement claim
  (UPDATE ... WHERE used_at IS NULL RETURNING) — single-use even under concurrent
  submission (L1 test fires 8 concurrent claims; exactly one wins).
- core::invite: generate (base58, 128-bit) + sha256 hash.
- enforce_invite in extension_register, mirroring enforce_pow (returning bypass;
  ONE literal error for not-found/used/expired — never an oracle).
- RegistrationConfig.require_invite + RegistrationCapability in /capabilities.
- smirk-admin mint-invite --count/--label (prints raw codes once; stores hashes).
- L1 tests (single-use + concurrency race), .env.example, OpenAPI (drift clean).

migrations/20260630000001_invite_codes.sql

@@ -0,0 +1,20 @@
+-- Operator-minted registration invite codes — one composable registration gate
+-- (alongside PoW and, later, pay-to-register). v1 is single-use, operator-minted
+-- via the `smirk-admin mint-invite` CLI; the schema leaves room for multi-use,
+-- expiry, and referral without a breaking change.
+--
+-- Only sha256(code) is stored. The raw code is a bearer secret shown once at
+-- mint time and never persisted, so a database leak is NOT a list of usable
+-- codes — codes are >=128-bit random, so the hash is not brute-forceable and no
+-- pepper is needed.
+CREATE TABLE invite_codes (
+    code_hash   TEXT PRIMARY KEY,                       -- hex sha256 of the raw code
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    label       TEXT,                                   -- optional operator note (batch / who-for)
+    expires_at  TIMESTAMPTZ,                            -- NULL = never (reserved; v1 mints without expiry)
+    used_at     TIMESTAMPTZ,                            -- NULL = unused (single-use)
+    used_by     UUID REFERENCES users (id) ON DELETE SET NULL
+);
+
+-- Fast lookup of still-spendable codes (mint listing / future sweeps).
+CREATE INDEX idx_invite_codes_unused ON invite_codes (created_at) WHERE used_at IS NULL;