feat(bootstrap,api,infra): scaffold app composition, health route, and postgres/minio/qdrant adapters

Why:
- first runnable slice of ADR-0012's resource-lifetime rules and ADR-0015's package layout: app-lifetime clients built once in the lifespan, released via explicit dependencies.

Changes:
- Settings split into per-domain nested settings (postgres/minio/ingestion/qdrant/app/logging); FastAPI app wired with /healthz, /readyz and a /v1 router; Postgres/MinIO/Qdrant adapters and SQLAlchemy models for tenants, API keys, source files, ingestion jobs/events.

src/infrastructure/postgres/models/base.py

@@ -0,0 +1,5 @@
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    pass