### Summary
The Go backend never seeded the `canvas_template` table, so the "Create
agent from template" page was blank when the frontend proxies to the Go
API (`API_PROXY_SCHEME=go`). This PR adds `SeedCanvasTemplates()` in
`internal/dao`, invoked from `InitDB()` after migrations, which loads
`agent/templates/*.json` and mirrors Python's `add_graph_templates()`
behavior.
Changes:
- Add `internal/dao/canvas_template_seed.go` to parse and upsert
built-in templates.
- Call `SeedCanvasTemplates()` in `InitDB()`.
- Add `CanvasTypes` (`JSONSlice`) to `entity.CanvasTemplate` so the
frontend can filter/group by category.
- Skip seeding gracefully when the templates directory is absent.
This fixes the blank template catalogue in Go mode.
