Pin versions, add resource limits, and harden config

- Pin all Docker image tags (mysql 8.0.40, python 3.12.13-slim, node 20.20.1-alpine, nginx 1.29.6-alpine)
- Pin all frontend npm dependencies to exact versions (remove ^ ranges)
- Add mem_limit and cpus resource limits to all three containers
- Add non-root appuser to backend Dockerfile
- Migrate JWT from python-jose to PyJWT
- Remove default admin_password in config.py — must be explicitly set in .env
- Add DOCS_ENABLED flag to config and .env.example (default false)
- Add indexes on session_date, is_active, event_type in session models
- Add limit/offset pagination to all log endpoints

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

backend/app/config.py

@@ -10,7 +10,8 @@ class Settings(BaseSettings):
     refresh_token_expire_days: int = 30
     cors_origins: str = "http://localhost:8054"
     admin_username: str = "admin"
-    admin_password: str = "change_me_admin_password"
+    admin_password: str  # no default — must be explicitly set in .env
+    docs_enabled: bool = False
 
     @property
     def cors_origins_list(self) -> list[str]: