fix(backend): fix file upload and integration test suite

- Make data.files.exif column nullable (was NOT NULL but service passes nil
  for files without EXIF data, causing a constraint violation on upload)
- FileRepo.Create: include id in INSERT so disk storage path and DB record
  share the same UUID (previously DB generated its own UUID, causing a mismatch)
- Integration test: use correct filter DSL format {t=<uuid>} instead of tag:<uuid>

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/003_data_tables.sql

@@ -50,7 +50,7 @@ CREATE TABLE data.files (
     content_datetime timestamptz  NOT NULL DEFAULT clock_timestamp(),  -- content datetime (e.g. photo taken)
     notes            text,
     metadata         jsonb,                 -- user-editable key-value data
-    exif             jsonb        NOT NULL DEFAULT '{}'::jsonb,  -- EXIF data extracted at upload (immutable)
+    exif             jsonb,                 -- EXIF data extracted at upload (immutable)
     phash            bigint,                -- perceptual hash for duplicate detection (future)
     creator_id       smallint     NOT NULL REFERENCES core.users(id)
                                   ON UPDATE CASCADE ON DELETE RESTRICT,