Implement security hardening across frontend, backend, and infrastructure

- nginx: add X-Content-Type-Options, X-Frame-Options, X-XSS-Protection,
  and Referrer-Policy headers on all responses; rate limit /api/auth/login
  to 5 req/min per IP (burst 3) to prevent brute force
- frontend: add escHtml() utility to api.js; use it on all notes fields
  across dashboard, log, history, flock, and budget pages to prevent XSS
- log.js: fix broken loadRecent() call referencing removed #recent-body
  element; replaced with loadHistory() from history.js
- schemas.py: raise minimum password length from 6 to 10 characters
- admin.py: add audit logging for password reset, disable, delete, and
  impersonate actions; fix impersonate to use named admin param for logging
- main.py: add startup env validation — exits with clear error if any
  required env var is missing; configure structured logging to stdout
- docker-compose.yml: add log rotation (10 MB / 3 files) to all services

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

backend/main.py

@@ -1,5 +1,6 @@
 import os
 import logging
+import sys
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
@@ -12,8 +13,21 @@ from auth import hash_password
 from routers import eggs, flock, feed, stats, other
 from routers import auth_router, admin
 
+logging.basicConfig(
+    level=logging.INFO,
+    format="%(asctime)s %(levelname)s %(name)s: %(message)s",
+    stream=sys.stdout,
+)
 logger = logging.getLogger("yolkbook")
 
+_REQUIRED_ENV = ["ADMIN_USERNAME", "ADMIN_PASSWORD", "JWT_SECRET", "DATABASE_URL"]
+
+def _validate_env():
+    missing = [k for k in _REQUIRED_ENV if not os.environ.get(k)]
+    if missing:
+        logger.critical("Missing required environment variables: %s", ", ".join(missing))
+        sys.exit(1)
+
 
 def _seed_admin():
     """Create or update the admin user from environment variables.
@@ -68,6 +82,7 @@ def _run_migrations():
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    _validate_env()
     Base.metadata.create_all(bind=engine)
     _run_migrations()
     _seed_admin()