Add PainterPalette integration, canonical DB schema, and influence UI fixes.

Integrate Inputs/PainterPalette.csv for artist metadata and influence links, add db/schema.sql with server/migrate.js, letterbox influence panel thumbnails, and refresh Titian/Pontormo images.

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

db/migrate-artist-palette.sql

@@ -0,0 +1,6 @@
+-- PainterPalette.csv enrichment fields on artists
+ALTER TABLE artists
+  ADD COLUMN IF NOT EXISTS palette_metadata JSONB;
+
+CREATE INDEX IF NOT EXISTS artists_palette_metadata_idx
+  ON artists USING gin (palette_metadata);