### 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):
