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/routers/admin.py

@@ -1,3 +1,5 @@
+import logging
+
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import select
 from sqlalchemy.orm import Session
@@ -8,6 +10,7 @@ from schemas import UserCreate, UserOut, ResetPasswordRequest, TokenResponse
 from auth import hash_password, create_access_token, get_current_admin, get_current_user
 
 router = APIRouter(prefix="/api/admin", tags=["admin"])
+logger = logging.getLogger("yolkbook")
 
 
 @router.get("/users", response_model=list[UserOut])
@@ -50,6 +53,7 @@ def reset_password(
         raise HTTPException(status_code=404, detail="User not found")
     user.hashed_password = hash_password(body.new_password)
     db.commit()
+    logger.warning("Admin '%s' reset password for user '%s' (id=%d).", current_admin.username, user.username, user.id)
     return {"detail": f"Password reset for {user.username}"}
 
 
@@ -66,6 +70,7 @@ def disable_user(
         raise HTTPException(status_code=400, detail="Cannot disable your own account")
     user.is_disabled = True
     db.commit()
+    logger.warning("Admin '%s' disabled user '%s' (id=%d).", current_admin.username, user.username, user.id)
     return {"detail": f"User {user.username} disabled"}
 
 
@@ -94,6 +99,7 @@ def delete_user(
         raise HTTPException(status_code=404, detail="User not found")
     if user.id == current_admin.id:
         raise HTTPException(status_code=400, detail="Cannot delete your own account")
+    logger.warning("Admin '%s' deleted user '%s' (id=%d).", current_admin.username, user.username, user.id)
     db.delete(user)
     db.commit()
 
@@ -101,11 +107,12 @@ def delete_user(
 @router.post("/users/{user_id}/impersonate", response_model=TokenResponse)
 def impersonate_user(
     user_id: int,
-    _: User = Depends(get_current_admin),
+    current_admin: User = Depends(get_current_admin),
     db: Session = Depends(get_db),
 ):
     user = db.get(User, user_id)
     if not user:
         raise HTTPException(status_code=404, detail="User not found")
     token = create_access_token(user.id, user.username, user.is_admin, user.timezone)
+    logger.warning("Admin '%s' (id=%d) is impersonating user '%s' (id=%d).", current_admin.username, current_admin.id, user.username, user.id)
     return TokenResponse(access_token=token)