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/main.py

@@ -0,0 +1,16 @@
+from fastapi import FastAPI
+
+from src.api.router import router as v1_router
+from src.api.routers.health import router as health_router
+from src.bootstrap.lifespan import create_lifespan
+from src.config import Settings
+
+
+def create_app(settings: Settings | None = None) -> FastAPI:
+    app = FastAPI(lifespan=create_lifespan(settings))
+    app.include_router(health_router)
+    app.include_router(v1_router, prefix="/v1")
+    return app
+
+
+app = create_app()