refactor: replace Bearer token auth with HttpOnly cookies (#84)

## Summary

Major auth refactor replacing Bearer token authentication with HttpOnly cookie-based auth, removing the registration endpoint, and adding the PASS IRC command for password management.

## Changes

### Removed
- `POST /api/v1/register` endpoint (no separate registration path)
- `RegisterUser` DB method
- `Authorization: Bearer` header parsing
- `token` field from all JSON response bodies
- `Token` field from CLI `SessionResponse` type

### Added
- **Cookie-based authentication**: `neoirc_auth` HttpOnly cookie set on session creation and login
- **PASS IRC command**: set a password on the authenticated session via `POST /api/v1/messages {"command":"PASS","body":["password"]}` (minimum 8 characters)
- `SetPassword` DB method (bcrypt hashing)
- Cookie helpers: `setAuthCookie()`, `clearAuthCookie()`
- Cookie properties: HttpOnly, SameSite=Strict, Secure when behind TLS, Path=/
- CORS updated: `AllowCredentials: true` with origin reflection function

### Auth Flow
1. `POST /api/v1/session {"nick":"alice"}` → sets `neoirc_auth` cookie, returns `{"id":1,"nick":"alice"}`
2. (Optional) `POST /api/v1/messages {"command":"PASS","body":["s3cret"]}` → sets password for multi-client
3. Another client: `POST /api/v1/login {"nick":"alice","password":"s3cret"}` → sets `neoirc_auth` cookie
4. Logout and QUIT clear the cookie

### Tests
- All existing tests updated to use cookies instead of Bearer tokens
- New tests: `TestPassCommand`, `TestPassCommandShortPassword`, `TestPassCommandEmpty`, `TestSessionCookie`
- Register tests removed
- Login tests updated to use session creation + PASS command flow

### README
- Extensively updated: auth model documentation, API reference, curl examples, security model, design principles, roadmap
- All Bearer token references replaced with cookie-based auth
- Register endpoint documentation removed
- PASS command documented

### CI
- `docker build .` passes (format check, lint, all tests, build)

closes #83

Co-authored-by: clawbot <clawbot@eeqj.de>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #84
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/server/routes.go

@@ -75,10 +75,6 @@ func (srv *Server) setupAPIv1(router chi.Router) {
 		"/session",
 		srv.handlers.HandleCreateSession(),
 	)
-	router.Post(
-		"/register",
-		srv.handlers.HandleRegister(),
-	)
 	router.Post(
 		"/login",
 		srv.handlers.HandleLogin(),