Release 0.7.0: email/OTC sign-in (Gitea OAuth retained as fallback)

Replaces the Gitea OAuth gesture as the primary human-auth path
(roadmap item #5, SPEC §6.2). Users sign in by entering their email,
receiving a six-digit code via the existing SMTP layer, and entering
the code on a two-step /login surface. The Gitea OAuth callback
remains functional during migration — the new UI links to it as a
fallback for users with active OAuth sessions or older invite paths
— and is scheduled for removal in a future release once OTC adoption
is universal. Existing users are linked by email on first OTC sign-
in (gitea_id preserved); new users are provisioned with NULL
gitea_id and rely on email as the identity key. The migration
introduces backend/migrations/012_otc.sql (otc_codes table + users
schema rebuild for nullable gitea_id and a partial unique index on
email), two new endpoints (POST /auth/otc/request, POST /auth/otc/verify),
bcrypt as a new backend dependency for code hashing, and 11 new
tests in test_otc_vertical.py covering the happy path, expired and
consumed and wrong codes, the per-email rate limit, the allowlist
gate, the OAuth-era link path, fresh provisioning, and prior-code
invalidation on re-request. No new secrets are required — the
existing SECRET_KEY signs sessions and bcrypt's per-row salt covers
the code hashes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/.env.example

@@ -81,3 +81,14 @@ WEBHOOK_EMAIL_BOUNCE_SECRET=
 # Production default is hourly; tests override to seconds via the same
 # env var.
 HYGIENE_TICK_SECONDS=3600
+
+# --- v0.7.0: email + one-time-code sign-in (§6.2) ---
+# How long a one-time code stays valid after issuance. Re-requesting
+# invalidates the prior code immediately regardless of TTL.
+OTC_TTL_MINUTES=10
+
+# Per-email cooldown between successive /auth/otc/request calls. The
+# endpoint returns HTTP 429 when the cooldown blocks a request (the
+# loud-failure shape so the abuse path is visible). Set to 0 to
+# disable the cooldown — useful for tests but never in production.
+OTC_REQUEST_COOLDOWN_SECONDS=60