feat: initialize Go module and implement domain layer

- Add go.mod (module tanabata/backend, Go 1.21) with uuid dependency
- Implement internal/domain: File, Tag, TagRule, Category, Pool, PoolFile,
  User, Session, Permission, ObjectType, AuditEntry + all pagination types
- Add domain error sentinels (ErrNotFound, ErrForbidden, etc.)
- Add context helpers WithUser/UserFromContext for JWT propagation
- Fix migration: remove redundant DEFAULT on exif jsonb column

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

backend/migrations/001_init.sql

@@ -164,7 +164,7 @@ 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        NOT NULL,  -- 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,