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/app/auth.py

@@ -193,10 +193,16 @@ def current_user(request: Request) -> SessionUser | None:
     ).fetchone()
     if row is None:
         return None
+    # v0.7.0: OTC-provisioned users have NULL gitea_id / gitea_login.
+    # Coerce nulls to the SessionUser's typed defaults so downstream
+    # code (Actor, _on_behalf_trailer) reads a stable shape regardless
+    # of which sign-in path the row came from. The DB remains the
+    # source of truth for "is this an OAuth-linked user" (gitea_id IS
+    # NOT NULL); the in-memory SessionUser is the per-request handle.
     return SessionUser(
         user_id=row["id"],
-        gitea_id=row["gitea_id"],
-        gitea_login=row["gitea_login"],
+        gitea_id=row["gitea_id"] or 0,
+        gitea_login=row["gitea_login"] or "",
         display_name=row["display_name"],
         email=row["email"] or "",
         avatar_url=row["avatar_url"] or "",