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/templates/base.html

@@ -52,6 +52,12 @@
               onclick="(function(){var d=document.documentElement;var t=d.dataset.theme==='light'?'dark':'light';d.dataset.theme=t;try{localStorage.setItem('cassandra.theme',t);}catch(e){}})()">
         <span class="theme-toggle__label"></span>
       </button>
+      {% set cu = request.state.current_user if request.state and request.state.current_user is defined else None %}
+      {% if cu and cu.user %}
+        <span class="user-chip">{{ cu.user.email }} · <a href="/logout">logout</a></span>
+      {% elif cu and cu.is_admin %}
+        <span class="user-chip">admin · <a href="/logout">logout</a></span>
+      {% endif %}
       <span class="meta">v0.1 · UTC</span>
     </div>
   </header>