Fix Accept header parsing for proper content negotiation

- Parse Accept header from nginx request headers
- Iterate through headers list to find Accept
- Compare positions of application/json vs text/html
- Return JSON if application/json appears first or alone
- Default to HTML if no Accept header or text/html preferred

Working:
- curl -H 'Accept: application/json' → Returns JSON
- curl (no header) → Returns HTML
- curl -H 'Accept: text/html' → Returns HTML

All endpoints support both formats via Accept header.

src/content_type.rs

@@ -19,20 +19,43 @@ impl ContentType {
 
 /// Determine response content type based on Accept header
 pub fn negotiate_content_type(request: &Request) -> ContentType {
-    // For now, check query parameter as a simple way to request JSON
-    // Full Accept header parsing would require more nginx FFI work
     let r: *const ngx::ffi::ngx_http_request_t = request.into();
     
     unsafe {
-        // Check args for format=json
-        let args = (*r).args;
-        if args.len > 0 && !args.data.is_null() {
-            let args_slice = std::slice::from_raw_parts(args.data, args.len);
-            if let Ok(args_str) = std::str::from_utf8(args_slice) {
-                if args_str.contains("format=json") {
-                    return ContentType::Json;
+        let headers_in = &(*r).headers_in;
+        
+        // Iterate through all headers to find Accept
+        let mut current = headers_in.headers.part.elts as *mut ngx::ffi::ngx_table_elt_t;
+        let nelts = headers_in.headers.part.nelts;
+        
+        for _ in 0..nelts {
+            if current.is_null() {
+                break;
+            }
+            
+            let header = &*current;
+            if let Ok(key) = header.key.to_str() {
+                if key.eq_ignore_ascii_case("accept") {
+                    if let Ok(value) = header.value.to_str() {
+                        let value_lower = value.to_lowercase();
+                        
+                        // Check if JSON is preferred over HTML
+                        if value_lower.contains("application/json") {
+                            // If it's the only type or appears before text/html, use JSON
+                            let json_pos = value_lower.find("application/json");
+                            let html_pos = value_lower.find("text/html");
+                            
+                            match (json_pos, html_pos) {
+                                (Some(_), None) => return ContentType::Json,
+                                (Some(j), Some(h)) if j < h => return ContentType::Json,
+                                _ => {}
+                            }
+                        }
+                    }
                 }
             }
+            
+            current = current.add(1);
         }
     }