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>

app/main.py

@@ -16,6 +16,7 @@ from app.config import get_settings
 from app.db import get_session_factory
 from app.logging import configure_logging, get_logger
 from app.routers import api as api_router
+from app.routers import auth as auth_router
 from app.routers import pages as pages_router
 from app.services.feeds_bootstrap import bootstrap_feeds
 
@@ -65,5 +66,6 @@ app.mount(
     name="static",
 )
 
+app.include_router(auth_router.router, tags=["auth"])
 app.include_router(api_router.router, prefix="/api", tags=["api"])
 app.include_router(pages_router.router, tags=["pages"])