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/store/store_test.go

@@ -38,12 +38,19 @@ func TestMigrateCreatesSchema(t *testing.T) {
 		}
 	}
 
+	// Derive the expected head version from the embedded files so adding a
+	// migration doesn't break this test.
+	migs, err := loadMigrations()
+	if err != nil {
+		t.Fatalf("loadMigrations: %v", err)
+	}
+	wantVersion := migs[len(migs)-1].version
 	var version int
 	if err := db.SQL().QueryRow(`SELECT max(version) FROM schema_migrations`).Scan(&version); err != nil {
 		t.Fatalf("read schema_migrations: %v", err)
 	}
-	if version != 2 {
-		t.Errorf("schema version = %d, want 2", version)
+	if version != wantVersion {
+		t.Errorf("schema version = %d, want %d", version, wantVersion)
 	}
 }
 
@@ -54,12 +61,16 @@ func TestMigrateIsIdempotent(t *testing.T) {
 	if err := db.Migrate(context.Background()); err != nil {
 		t.Fatalf("second Migrate: %v", err)
 	}
+	migs, err := loadMigrations()
+	if err != nil {
+		t.Fatalf("loadMigrations: %v", err)
+	}
 	var count int
 	if err := db.SQL().QueryRow(`SELECT count(*) FROM schema_migrations`).Scan(&count); err != nil {
 		t.Fatalf("count schema_migrations: %v", err)
 	}
-	if count != 2 {
-		t.Errorf("schema_migrations rows = %d, want 2", count)
+	if count != len(migs) {
+		t.Errorf("schema_migrations rows = %d, want %d", count, len(migs))
 	}
 }