feat: model selection+visibility, better filenames, picdrop direct upload + configurable gallery

- studio: quality/model selector (friendly names from active models); library shows model used
- seed 2 models (Schnell/Beste Qualitaet); GET /api/models for studio+library
- filename: YYYY-MM-DD_motif_format.png (was redundant slug, no date)
- picdrop: direct upload to final name (fix .tmp- leftover from failed rename on picdrop); cleanupTmp helper
- configurable default gallery (migration 003, settings + admin field); base_path=SFTP root, gallery=subfolder

migrations/003_settings_gallery.sql

@@ -0,0 +1,2 @@
+-- Konfigurierbare Standard-Picdrop-Galerie (statt fest verdrahtet).
+ALTER TABLE settings ADD COLUMN IF NOT EXISTS picdrop_default_gallery text;