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/errors.py

@@ -0,0 +1,17 @@
+"""Upload-validation failures (ADR-0008). No HTTP knowledge here —
+`src/api/errors.py` maps these to status codes.
+"""
+
+
+class FilesError(Exception):
+    """Base class for file-upload failures."""
+
+
+class InvalidUploadError(FilesError):
+    """Missing domain, empty file, or content that doesn't match its
+    declared extension. Maps to `400`.
+    """
+
+
+class FileTooLargeError(FilesError):
+    """The upload exceeds `INGESTION_MAX_UPLOAD_SIZE_MB`. Maps to `413`."""