feat: support multiple canvas_types for agent templates and remove duplicate files (#14030)

### What problem does this PR solve?

Closes #13907

The template catalog had duplicate files (e.g. `*_r.json`) only to place
the same template into multiple sidebar groups.
This increases maintenance cost and makes template updates error-prone.

This PR adds first-class support for multiple template categories in a
single file via `canvas_types`, then removes duplicate template files.

What changed:
- Added `canvas_types` to `CanvasTemplate` model and DB migration.
- Added normalization logic when loading templates:
  - accepts legacy `canvas_type`
  - accepts new `canvas_types`
  - merges/deduplicates values
- preserves backward compatibility by keeping `canvas_type` as first
normalized value.
- Updated template import flow to load only `.json` files and in stable
sorted order.
- Updated frontend template filtering to match on `canvas_types` first,
with fallback to legacy `canvas_type`.
- Consolidated duplicated template pairs into single files and removed:
  - `deep_search_r.json`
  - `reflective_academic_paper_generator_r.json`
  - `seo_article_writer_r.json`
- Added regression/edge-case tests for category normalization and route
serialization expectations.

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

test/unit_test/api/db/test_template_utils.py

@@ -0,0 +1,66 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+import pytest
+
+from api.db.template_utils import normalize_canvas_template_categories
+
+
+@pytest.mark.p2
+def test_normalize_canvas_template_categories_legacy_canvas_type():
+    payload = {"id": 1, "canvas_type": "Recommended"}
+
+    normalized = normalize_canvas_template_categories(payload)
+
+    assert normalized["canvas_type"] == "Recommended"
+    assert normalized["canvas_types"] == ["Recommended"]
+
+
+@pytest.mark.p2
+def test_normalize_canvas_template_categories_with_canvas_types_only():
+    payload = {
+        "id": 1,
+        "canvas_types": ["Recommended", "Agent", "Agent", "  ", 1, None],
+    }
+
+    normalized = normalize_canvas_template_categories(payload)
+
+    assert normalized["canvas_type"] == "Recommended"
+    assert normalized["canvas_types"] == ["Recommended", "Agent"]
+
+
+@pytest.mark.p2
+def test_normalize_canvas_template_categories_merges_legacy_and_new_field():
+    payload = {
+        "id": 1,
+        "canvas_type": "Marketing",
+        "canvas_types": ["Recommended", "Marketing", "Agent"],
+    }
+
+    normalized = normalize_canvas_template_categories(payload)
+
+    assert normalized["canvas_type"] == "Marketing"
+    assert normalized["canvas_types"] == ["Marketing", "Recommended", "Agent"]
+
+
+@pytest.mark.p2
+def test_normalize_canvas_template_categories_no_valid_categories():
+    payload = {"id": 1, "canvas_type": "   ", "canvas_types": [None, 3, "  "]}
+
+    normalized = normalize_canvas_template_categories(payload)
+
+    assert normalized["canvas_type"] is None
+    assert normalized["canvas_types"] == []