Spool Upload Endpoint

Service-to-service spool: sibling copilots (unity-copilot first) POST an oversize tool result here as NDJSON text; we forward it to analyze-mcp's /uploads (same short-TTL proxy JWT as backend/app/uploads.py's /api/uploads) and hand back analyze's file sidecar (file_id, columns, sample, ...) verbatim, so the caller can hand the agent a file_id instead of the raw rows. HMAC-gated like the other /internal/* routes in this module — but HMAC alone only proves "a sibling service", not "for THIS tenant": any HMAC-authenticated caller could otherwise assert an arbitrary tenant_id, a durable cross-tenant write. So the caller MUST also present, as a Bearer token, the SAME pozeidon-signed JWT it was handed for its live MCP request (minted by mint_tenant_jwt) — proof-of-tenant, verified against our own signing keys (see auth.verify_tenant_proof_jwt) with its tenant_id claim required to exactly match the body's tenant_id.