feat: enhance security with IP banning, ownership checks, and SSRF protection

Add comprehensive security improvements across the application:

- **IP-based login protection**: Implement IP ban system tracking login failures, auto-banning after threshold exceeded
- **Ownership verification**: Add userID parameter to Delete/Update operations for posts and comments to prevent unauthorized modifications
- **SSRF protection**: Add URL and resolved host validation for image URLs in chat and OpenAI client
- **SQL injection prevention**: Add EscapeLikeWildcard utility to escape special characters in LIKE queries
- **HTTP security**: Configure server timeouts and add security headers middleware
- **Rate limiting**: Refactor to support configurable duration and per-endpoint rate limits for auth routes
- **Error handling**: Standardize error responses using HandleError and proper error types

internal/middleware/security_headers.go

@@ -0,0 +1,20 @@
+package middleware
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+func SecurityHeaders() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Header("Content-Security-Policy", "default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self'; connect-src 'self' https:; frame-ancestors 'none'; base-uri 'self'; form-action 'self'")
+		c.Header("Referrer-Policy", "strict-origin-when-cross-origin")
+		c.Header("Permissions-Policy", "camera=(), microphone=(), geolocation=(), payment=()")
+		c.Header("X-Content-Type-Options", "nosniff")
+		c.Header("X-Frame-Options", "DENY")
+		c.Header("X-XSS-Protection", "0")
+		c.Header("Cross-Origin-Opener-Policy", "same-origin")
+		c.Header("Cross-Origin-Resource-Policy", "same-origin")
+
+		c.Next()
+	}
+}