feat: safe deletion, album/asset delete, trash, README update

- volume-aware deletion: read-only volumes remove DB only, writable
  volumes soft-delete to trash with configurable grace period
- trash page with restore, worker purge sweep (TRASH_RETENTION_DAYS)
- album delete endpoint + sidebar trash icon
- asset delete from timeline selection toolbar
- all listing queries exclude trashed assets (deleted_at IS NULL)
- timeline ordered by EXIF capture date, date-summary endpoint
- README rewritten with features, setup, full env var table

crates/adapters/postgres/migrations/018_soft_delete.sql

@@ -0,0 +1,3 @@
+ALTER TABLE assets ADD COLUMN deleted_at TIMESTAMPTZ NULL;
+ALTER TABLE assets ADD COLUMN deleted_by UUID NULL REFERENCES users(id);
+CREATE INDEX idx_assets_deleted ON assets (deleted_at) WHERE deleted_at IS NOT NULL;