feat(frontend): T46 configure HTMX and CSRF protection

- Add CSRFMiddleware for form protection
- Implement token generation and validation
- Add CSRF meta tag to base.html
- Create tests for CSRF protection

Tests: 13 passing

src/openrouter_monitor/middleware/csrf.py

@@ -0,0 +1,132 @@
+"""CSRF Protection Middleware.
+
+Provides CSRF token generation and validation for form submissions.
+"""
+import secrets
+from typing import Optional
+
+from fastapi import Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware
+
+
+class CSRFMiddleware(BaseHTTPMiddleware):
+    """Middleware for CSRF protection.
+    
+    Generates CSRF tokens for sessions and validates them on
+    state-changing requests (POST, PUT, DELETE, PATCH).
+    """
+    
+    CSRF_TOKEN_NAME = "csrf_token"
+    CSRF_HEADER_NAME = "X-CSRF-Token"
+    SAFE_METHODS = {"GET", "HEAD", "OPTIONS", "TRACE"}
+    
+    def __init__(self, app, cookie_name: str = "csrf_token", cookie_secure: bool = False):
+        super().__init__(app)
+        self.cookie_name = cookie_name
+        self.cookie_secure = cookie_secure
+    
+    async def dispatch(self, request: Request, call_next):
+        """Process request and validate CSRF token if needed.
+        
+        Args:
+            request: The incoming request
+            call_next: Next middleware/handler in chain
+            
+        Returns:
+            Response from next handler
+        """
+        # Generate or retrieve CSRF token
+        csrf_token = self._get_or_create_token(request)
+        
+        # Validate token on state-changing requests
+        if request.method not in self.SAFE_METHODS:
+            is_valid = await self._validate_token(request, csrf_token)
+            if not is_valid:
+                from fastapi.responses import JSONResponse
+                return JSONResponse(
+                    status_code=403,
+                    content={"detail": "CSRF token missing or invalid"}
+                )
+        
+        # Store token in request state for templates
+        request.state.csrf_token = csrf_token
+        
+        # Process request
+        response = await call_next(request)
+        
+        # Set CSRF cookie
+        response.set_cookie(
+            key=self.cookie_name,
+            value=csrf_token,
+            httponly=False,  # Must be accessible by JavaScript
+            secure=self.cookie_secure,
+            samesite="lax",
+            max_age=3600 * 24 * 7,  # 7 days
+        )
+        
+        return response
+    
+    def _get_or_create_token(self, request: Request) -> str:
+        """Get existing token from cookie or create new one.
+        
+        Args:
+            request: The incoming request
+            
+        Returns:
+            CSRF token string
+        """
+        # Try to get from cookie
+        token = request.cookies.get(self.cookie_name)
+        if token:
+            return token
+        
+        # Generate new token
+        return secrets.token_urlsafe(32)
+    
+    async def _validate_token(self, request: Request, expected_token: str) -> bool:
+        """Validate CSRF token from request.
+        
+        Checks header first, then form data.
+        
+        Args:
+            request: The incoming request
+            expected_token: Expected token value
+            
+        Returns:
+            True if token is valid, False otherwise
+        """
+        # Check header first (for HTMX/ajax requests)
+        token = request.headers.get(self.CSRF_HEADER_NAME)
+        
+        # If not in header, check form data
+        if not token:
+            try:
+                # Parse form data from request body
+                body = await request.body()
+                if body:
+                    from urllib.parse import parse_qs
+                    form_data = parse_qs(body.decode('utf-8'))
+                    if b'csrf_token' in form_data:
+                        token = form_data[b'csrf_token'][0]
+            except Exception:
+                pass
+        
+        # Validate token
+        if not token:
+            return False
+        
+        return secrets.compare_digest(token, expected_token)
+
+
+def get_csrf_token(request: Request) -> Optional[str]:
+    """Get CSRF token from request state.
+    
+    Use this in route handlers to pass token to templates.
+    
+    Args:
+        request: The current request
+        
+    Returns:
+        CSRF token or None
+    """
+    return getattr(request.state, "csrf_token", None)