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()

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)

backend/schemas.py

@@ -16,10 +16,10 @@ class TokenResponse(BaseModel):
 
 class ChangePasswordRequest(BaseModel):
     current_password: str
-    new_password: str = Field(min_length=6)
+    new_password: str = Field(min_length=10)
 
 class ResetPasswordRequest(BaseModel):
-    new_password: str = Field(min_length=6)
+    new_password: str = Field(min_length=10)
 
 class TimezoneUpdate(BaseModel):
     timezone: str = Field(min_length=1, max_length=64)
@@ -29,7 +29,7 @@ class TimezoneUpdate(BaseModel):
 
 class UserCreate(BaseModel):
     username: str = Field(min_length=2, max_length=64)
-    password: str = Field(min_length=6)
+    password: str = Field(min_length=10)
 
 class UserOut(BaseModel):
     id:          int