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.
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