FILE datatype: heap references with cold-backend blob storage

[jamals86]

Context

docs/roadmap.md lists FILE datatype as near-term priority #5. This is a KalamDB-style column type for binary/file payloads: the PostgreSQL heap stores a compact reference; the blob lives in the table’s registered cold backend (filesystem or object store).

Goal

• Introduce a FILE (or equivalent) column type / storage contract for managed tables
• Upload and fetch APIs read/write through the registered cold backend rather than embedding large values in row storage
• Keep references durable across flush, backup/export, and scoped paths

Acceptance

• Design note: type OID / SQL surface, reference format, GC / orphan rules, interaction with manage/unmanage
• Heap stores compact reference only; blob bytes live in cold storage under the table’s storage binding
• Upload + fetch APIs (SQL and/or documented operator path) with RLS/scope fail-closed for user-typed tables
• Flush / demigrate / backup behavior defined (references remain valid; blobs included in export where required)
• Tests for upload, fetch, delete/orphan cleanup, and scoped access denial
• Docs in SQL API + roadmap

References

• docs/roadmap.md § FILE datatype
• Related: scoped storage, backup/export (Expose operator SQL: changes_since, backup_manifest, validate_cold_storage, EXPORT/IMPORT #56)