Add sharing backend: shares CRUD + ACL enforcement everywhere (#16)

- domain: Garden gains a computed MyRole ("owner"/"editor"/"viewer"); new
  ShareWithUser (share + recipient identity); sentinels ErrShareUserNotFound,
  ErrCannotShareWithSelf, ErrShareExists.
- store/shares.go: GetShareRole, ListSharesForGarden (joined with users),
  CreateShare (UNIQUE → ErrShareExists), UpdateShareRole, DeleteShare.
- store/gardens.go: ListGardensForActor returns owned + shared-with-me gardens,
  each carrying my_role (replaces the owner-only list).
- service: requireGardenRole now consults garden_shares (owner implicit, else the
  share's role, else masked ErrNotFound) and stamps MyRole on every read-through.
  UpdateGarden tightened to OWNER-only (editors edit contents, not the garden).
  UpdatePlanting only re-checks plant visibility when the plant is CHANGED, so a
  shared editor can edit a plop that uses the owner's private plant.
- service/shares.go: ListShares/AddShare/UpdateShareRole (owner only), RemoveShare
  (owner or a recipient leaving). AddShare targets an existing account by exact
  email; unknown → 404, self → 400, duplicate → 409.
- api: GET,POST /gardens/:id/shares and PATCH,DELETE /gardens/:id/shares/:userId.

Tests: the full {owner, editor, viewer, stranger} × {read, mutate object, mutate
planting, edit garden, delete garden, manage shares} ACL matrix; shared gardens
carry my_role; AddShare error cases; role upgrade + self-leave; plus a two-user
HTTP flow. The matrix caught the editor-cannot-edit-owner's-plant bug above.

GOWORK=off go build/vet/test ./internal/... green.

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

internal/service/plantings.go

@@ -122,8 +122,16 @@ func (s *Service) UpdatePlanting(ctx context.Context, actorID, plantingID int64,
 	}
 
 	applyPlantingPatch(pl, patch)
-	// The plant may have changed; the (possibly new) plant must be visible.
-	plant, err := s.visiblePlant(ctx, actorID, pl.PlantID)
+	// The plant must be visible to the actor only when they're CHANGING it — an
+	// existing plop may reference a plant the actor can't see (e.g. a shared
+	// editor working in the owner's garden with the owner's private plant), and
+	// editing its radius/label/date must still work.
+	if patch.PlantID != nil {
+		if _, err := s.visiblePlant(ctx, actorID, pl.PlantID); err != nil {
+			return nil, err
+		}
+	}
+	plant, err := s.store.GetPlant(ctx, pl.PlantID) // for the derived count
 	if err != nil {
 		return nil, err
 	}