Add plant catalog backend: CRUD + seeded built-ins (#12)

Extends the plants store (which had only the /full read side) with full
catalog CRUD, adds a service layer with visibility/immutability rules, REST
endpoints, and a seed migration of ~32 built-ins.

- 0003_seed_plants.sql: 32 built-in plants (owner_id NULL, read-only), seeded
  once by the version-tracked migration runner.
- store: ListPlantsForActor (built-ins + own), Get/Create/Update/Delete +
  CountPlantingsForPlant; isForeignKeyViolation backstop for the RESTRICT FK.
- service: built-ins are read-only (ErrForbidden); another user's plants are
  invisible (ErrNotFound); delete of a referenced plant → ErrPlantInUse (409);
  validation mirrors the schema (category enum, spacing>0, hex color, icon).
- api: GET,POST /plants and PATCH,DELETE /plants/:id with the version guard.
- Made the migration-count store tests derive their expectation from the files
  so future migrations don't break them.

Service + API tests cover visibility, built-in immutability, validation,
version conflict, and delete-in-use.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/api/api.go

@@ -90,6 +90,13 @@ func New(cfg *config.Config, svc *service.Service) *gin.Engine {
 	objects.PATCH("/:id", h.updateObject)
 	objects.DELETE("/:id", h.deleteObject)
 
+	// Plant catalog: built-ins (seeded, read-only) plus the actor's own rows.
+	plants := v1.Group("/plants", h.requireAuth())
+	plants.GET("", h.listPlants)
+	plants.POST("", h.createPlant)
+	plants.PATCH("/:id", h.updatePlant)
+	plants.DELETE("/:id", h.deletePlant)
+
 	return r
 }