fix(server,flutter): sync wire format + systemVariant column (#357)

Two fixes in one commit because they're entangled — the systemVariant
work would have been theater otherwise.

## The wire-format bug

/api/library/sync was emitting PascalCase JSON for artist / album /
track / playlist upserts (raw json.Marshal of sqlc-generated structs
with no JSON tags — sqlc.yaml: emit_json_tags=false). Flutter's
sync_controller _*FromJson reads snake_case keys, so all metadata
sync rows landed in drift with empty strings / zero ints.

The like_track / like_album / like_artist / playlist_track entities
work because they're hand-built `map[string]string` payloads with
snake_case keys — they sidestepped the bug. The 4 raw-marshal
entities did not.

Existing sync test caught zero of this — it asserts on len(upserts)
not field shape.

Fix: server-side view structs in library_sync_views.go with proper
JSON tags + pgtype-flattening (UUID → 8-4-4-4-12 hex string,
Date → "2006-01-02"). Mirrors the playlistRowView pattern from
/api/playlists. New library_sync_views_test.go pins the wire keys
so future field-name drift breaks loud.

## systemVariant column (closes #357 plan C v1 limitation)

playlistSyncView now carries `system_variant` server → wire.

Flutter drift schema bumped from 1 → 2 with onUpgrade adding the
`systemVariant TEXT NULL` column to cached_playlists. Cursor reset
to 0 in the migration so existing rows refresh with the new field
on the next sync.

playlistsListProvider now filters locally by systemVariant:
- kind='user'   → systemVariant IS NULL  (the add-to-playlist sheet's intent)
- kind='system' → systemVariant IS NOT NULL
- kind='all'    → no filter

Closes the documented v1 limitation where the add-to-playlist sheet
showed system playlists alongside user-created ones.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/library_sync.go

@@ -141,7 +141,7 @@ func (h *handlers) hydrateUpserts(
 			return nil, err
 		}
 		for _, a := range artists {
-			b, _ := json.Marshal(a)
+			b, _ := json.Marshal(toArtistSyncView(a))
 			out["artist"] = append(out["artist"], b)
 		}
 	}
@@ -152,7 +152,7 @@ func (h *handlers) hydrateUpserts(
 			return nil, err
 		}
 		for _, a := range albums {
-			b, _ := json.Marshal(a)
+			b, _ := json.Marshal(toAlbumSyncView(a))
 			out["album"] = append(out["album"], b)
 		}
 	}
@@ -163,7 +163,7 @@ func (h *handlers) hydrateUpserts(
 			return nil, err
 		}
 		for _, t := range tracks {
-			b, _ := json.Marshal(t)
+			b, _ := json.Marshal(toTrackSyncView(t))
 			out["track"] = append(out["track"], b)
 		}
 	}
@@ -191,7 +191,7 @@ func (h *handlers) hydrateUpserts(
 			if syncpkg.FormatUUID(p.UserID) != userIDStr && !p.IsPublic {
 				continue
 			}
-			b, _ := json.Marshal(p)
+			b, _ := json.Marshal(toPlaylistSyncView(p))
 			out["playlist"] = append(out["playlist"], b)
 		}
 	}