Add user accounts: registration, login, and JWT auth

The first authenticated-user concept in Aurora Core -- everything else in
this codebase is public, read-only catalog data. Foundation for ratings
and a watchlist (not yet built): POST /auth/register, POST /auth/login,
GET /auth/me.

Security specifics worth knowing:
- Passwords hashed with argon2 (via argon2-cffi), never stored plain or
  logged.
- Login returns an identical 401 for a nonexistent email and a wrong
  password on a real one, including matching response timing
  (burn_time_like_a_password_check) -- otherwise either the status code
  or the timing alone lets a client enumerate registered emails.
- AURORA_SECRET_KEY now actually signs JWTs (previously defined but
  unused anywhere) -- .env.example's existing token_urlsafe(48) guidance
  already produces a long enough key, just flagged its new stakes.
- Email matched case-insensitively via explicit normalization in the
  service layer, not relied on from Pydantic's EmailStr (which does not
  lowercase the local part).

12 new integration tests (register/login/me over real HTTP via the
`client` fixture) plus 10 unit tests for the hashing/JWT logic itself,
following the same-day lesson that only unit tests actually execute
locally without Postgres -- the auth core is tested for real here rather
than only through integration tests that might not run before a deploy.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

.env.example

@@ -21,7 +21,9 @@ REDIS_URL=redis://localhost:6379/0
 REDIS_REQUIRED=false
 
 # --- Security ---
-# Generate with: python -c "import secrets; print(secrets.token_urlsafe(48))"
+# Signs user-account JWT access tokens (app/core/auth.py) -- a weak or
+# leaked value lets anyone forge a valid login for any user id. Generate
+# with: python -c "import secrets; print(secrets.token_urlsafe(48))"
 AURORA_SECRET_KEY=changeme-generate-a-real-secret
 # Comma-separated API keys allowed to call /api/v1/admin/* endpoints.
 AURORA_ADMIN_API_KEYS=changeme-admin-key