feat: session auth + per-user/persona UI at /{user}/{persona}

Replaces nginx basic auth with a proper per-user session system:

- auth_utils.py: bcrypt password hashing, JWT cookie creation/decode
- auth_middleware.py: validates JWT cookie on all routes except /login,
  /health, /static/, and webhook endpoints (/channels/, /webhook/)
- routers/ui.py: GET /login, POST /login, POST /logout,
  GET /{username}/{persona} — serves index.html with CORTEX_CONFIG injected
- static/login.html: minimal login form (dark theme, matches UI)
- main.py: registers SessionAuthMiddleware + ui.router
- config.py: jwt_secret, jwt_expire_days settings
- manage_passwords.py: CLI tool to set/check/list user passwords
- app.js: reads window.CORTEX_CONFIG (user + persona), sends both on
  every /chat and /orchestrate request; persona name shown in header;
  logout button (⏏) added to header
- requirements.txt: bcrypt, PyJWT, python-multipart
- .env.default: JWT_SECRET, JWT_EXPIRE_DAYS documented
- tests: client fixture injects JWT cookie; security test assertions
  updated for URL-normalized path traversal paths (still secure, codes differ)

All 80 tests pass.

Setup for a new user:
  python manage_passwords.py set scott
  python manage_passwords.py set holly

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/config.py

@@ -68,6 +68,10 @@ class Settings(BaseSettings):
     memory_budget_mid: int = 2000
     memory_budget_short: int = 3000
 
+    # Session auth
+    jwt_secret: str = "change-me-in-dotenv"   # override in .env: JWT_SECRET=<random>
+    jwt_expire_days: int = 30
+
     host: str = "0.0.0.0"
     port: int = 8000