Add custom source input for manually adding communities

- Add 'Custom Source' text input field to polling form
- Allows manual entry of subreddits (r/subreddit) or RSS URLs
- Custom input overrides dropdown selection if filled
- Dropdown becomes optional when custom source is entered
- Backend prioritizes custom_source_id over dropdown source_id

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

templates/admin_polling.html

@@ -223,6 +223,13 @@
                     <select class="form-select" name="source_id" id="source_id" required onchange="updateDisplayName()">
                         <option value="">Select source...</option>
                     </select>
+                    <p class="help-text">Or enter custom source below</p>
+                </div>
+
+                <div class="form-group">
+                    <label class="form-label" for="custom_source_id">Custom Source (optional)</label>
+                    <input type="text" class="form-input" name="custom_source_id" id="custom_source_id" placeholder="e.g., r/technology or https://example.com/feed.xml">
+                    <p class="help-text">For Reddit: r/subreddit | For RSS: full URL | Leave blank to use dropdown</p>
                 </div>
 
                 <div class="form-group">
@@ -370,6 +377,16 @@
                 displayNameInput.value = selectedOption.dataset.displayName;
             }
         }
+
+        // Handle custom source input - make dropdown optional when custom is filled
+        document.getElementById('custom_source_id').addEventListener('input', function() {
+            const sourceSelect = document.getElementById('source_id');
+            if (this.value.trim()) {
+                sourceSelect.removeAttribute('required');
+            } else {
+                sourceSelect.setAttribute('required', 'required');
+            }
+        });
     </script>
 </body>
 </html>