feat(files): add source-file upload with MinIO storage and Postgres repositories

Why:
- Implements plan 001 Phase 3's upload orchestration.

Changes:
- ObjectStorage port and MinIO adapter, thread-offloaded per ADR-0017.
- Tenant-scoped repositories for api_keys, source_files, ingestion_jobs.
- upload_source_file implementing the two-transaction shape with
  (tenant_id, domain, content_sha256) idempotency.

Impact:
- This phase stores bytes only -- chunks_indexed is always 0 until
  Phase 4/5 add parsing/embedding.

src/application/files/__init__.py

@@ -0,0 +1,19 @@
+"""Source-file upload and status use cases (ADR-0008, ADR-0009, ADR-0017)."""
+
+from src.application.files.errors import FilesError, FileTooLargeError, InvalidUploadError
+from src.application.files.models import UploadResult, ValidatedUpload
+from src.application.files.status import FileStatusResult, get_file_status
+from src.application.files.upload import upload_source_file
+from src.application.files.validation import validate_and_hash_upload
+
+__all__ = [
+    "FileStatusResult",
+    "FileTooLargeError",
+    "FilesError",
+    "InvalidUploadError",
+    "UploadResult",
+    "ValidatedUpload",
+    "get_file_status",
+    "upload_source_file",
+    "validate_and_hash_upload",
+]