phase A: user accounts + session-cookie auth

Replaces the static bearer-token gate with a real auth boundary. The
existing CASSANDRA_TOKEN path is retained as an admin / scripting escape
hatch — kept compatible by aliasing require_token to require_auth.

- New users table (migration 0007): email, argon2 password_hash, tier,
  email_verified (declared but not enforced until phase E), settings_json
  for the tone/analysis/anchor knobs we'll wire in phase D.
- app/services/auth_service.py: argon2-cffi password hashing with timing-
  attack-resistant authenticate() (always runs a hash verify even on
  unknown-email to deny a username-enumeration oracle).
- app/auth.py rewritten: require_auth returns a CurrentUser with either
  is_admin=True (bearer path) or a User object (session path). Failing
  requests get 303 → /login for HTML, 401 for API. Sessions signed with
  itsdangerous against CASSANDRA_SESSION_SECRET; 14-day TTL.
- app/routers/auth.py: /login, /signup, /logout. Login form preserves the
  ?next=… param for redirect-after-login. Signup respects a new
  CASSANDRA_SIGNUP_ENABLED flag.
- Standalone /login + /signup templates (no app chrome). base.html grows
  a user chip + logout link in the header (reads request.state.current_user).

Phase A's main known limitations are documented in the plan: email
verification is declared but not enforced; session revocation is
best-effort (cookie-only, not DB-backed). Both land in phase E.

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

pyproject.toml

@@ -17,6 +17,9 @@ dependencies = [
     "apscheduler>=3.10",
     "tenacity>=9.0",
     "structlog>=24.4",
+    "argon2-cffi>=23.1",
+    "itsdangerous>=2.2",
+    "email-validator>=2.2",
 ]
 
 [project.optional-dependencies]