Add movement artist filter modal and fix large-hall texture blanks

Clicking a movement opens ArtistFilterModal to choose artists before the gallery. Large halls no longer permanently blank frames when texture loads exceed the overlay deadline; fall back thumb to full to on-demand API.

Co-authored-by: Cursor <cursoragent@cursor.com>

Documentation/API.md

@@ -373,9 +373,38 @@ Artists belonging to a single movement.
 
 ---
 
+## `GET /api/movements/:id/artists-summary`
+
+Lightweight artist list for the **movement gallery entry filter** modal (portraits + painting counts).
+
+**Response** — array of:
+
+```json
+{
+  "id": 6,
+  "name": "Andrei Rublev",
+  "birth_year": 1360,
+  "death_year": 1430,
+  "portrait_path": "portraits/...",
+  "portrait_thumb_path": "portraits/thumbs/...",
+  "portrait_cache_key": 1710000000000,
+  "portrait_thumb_cache_key": 1710000000000,
+  "painting_count": 12
+}
+```
+
+| Field | Meaning |
+|-------|---------|
+| `painting_count` | Number of paintings by this artist (any movement filter is by `artists.movement_id`) |
+| `portrait_*_cache_key` | File mtimes for cache-busting (from `enrichArtistRow`) |
+
+Ordered by `birth_year`, then name. Localized when `?lang=` / locale headers request a non-default locale.
+
+---
+
 ## `GET /api/movements/:id/gallery`
 
-Full **movement gallery** payload for the 3D movement wings view (opened by clicking a movement name on the home page).
+Full **movement gallery** payload for the 3D movement wings view (opened by clicking a movement name on the home page, after the artist-filter modal).
 
 **Response**