feat(server): system collections become permanent — soft-delete + in-place restore + query_type-keyed provisioning

The old model hard-deleted system collections and re-created them at
the tail, resurrecting stale dedups and breaking ordering. Redesign
(user-directed B-final):

- schema.sql: collections.deleted_at TIMESTAMPTZ (idempotent ALTER —
  the whole schema is re-applied on every boot via executeSchema).
- queries.sql: every collection list/read query filters
  deleted_at IS NULL; new GetSystemCollectionByQueryType :one
  (unfiltered, sees tombstones), TombstoneSystemCollection :exec,
  ClearCollectionItems :exec, RestoreSystemCollectionRow :one
  (in-place metadata reset + un-tombstone; collections have no
  updated_at column — ResetUpdatedAt was a 500 caught by tests).
  sqlc regenerated.
- collection_service.DeleteCollection: system collections are
  tombstoned (preserving position/ownership); user collections keep
  the hard delete. No ErrNoRows to the client (delete is idempotent).
- dashboard_service.RestoreSystemCollection: resolve by query_type;
  found → in-place restore (ClearCollectionItems + metadata reset,
  un-tombstoned); missing → provision fresh. "Want to Read"/etc.
  stay at their home positions forever.
- handlers/collections.CreateCollection: name-taken is 409, not 500.
- handlers/auth.CreateDefaultCollectionsForUser: idempotent by
  (user_id, query_type) — update-in-place when a row for that query
  type exists (heals renames/tombstones), insert when missing; this
  also fixes the duplicate-system-collections regression at source.

internal/handlers/auth.go

@@ -1091,7 +1091,27 @@ func (h *AuthHandler) CreateDefaultCollectionsForUser(ctx context.Context, userI
 	}
 
 	for _, col := range defaultCollections {
-		_, err := h.db.CreateSystemCollection(ctx, database.CreateSystemCollectionParams{
+		// Idempotent by the STABLE identity (user + query_type): a
+		// preserved or renamed row heals to canonical metadata instead of
+		// duplicating (the naming-drift bug). Insert only when absent.
+		existing, err := h.db.GetSystemCollectionByQueryType(ctx, database.GetSystemCollectionByQueryTypeParams{
+			UserID:    userID,
+			QueryType: pgtype.Text{String: col.QueryType, Valid: true},
+		})
+		if err == nil {
+			if _, uerr := h.db.RestoreSystemCollectionRow(ctx, database.RestoreSystemCollectionRowParams{
+				ID:          existing.ID,
+				Name:        col.Name,
+				Description: pgtype.Text{String: col.Description, Valid: true},
+				Icon:        pgtype.Text{String: col.Icon, Valid: true},
+				Color:       pgtype.Text{String: col.Color, Valid: true},
+				Priority:    pgtype.Int4{Int32: col.Priority, Valid: true},
+			}); uerr != nil {
+				return uerr
+			}
+			continue
+		}
+		_, err = h.db.CreateSystemCollection(ctx, database.CreateSystemCollectionParams{
 			UserID:          userID,
 			Name:            col.Name,
 			Description:     pgtype.Text{String: col.Description, Valid: true},

internal/handlers/collections.go

@@ -111,6 +111,9 @@ func (h *CollectionHandler) CreateCollection(c *echo.Context) error {
 		req.ViewSettings,
 	)
 	if err != nil {
+		if errors.Is(err, services.ErrCollectionNameTaken) {
+			return c.JSON(http.StatusConflict, map[string]string{"error": "a collection with this name already exists"})
+		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}