feat: persist Heddle compaction archives in Supabase

[roackb2]

Summary

• upgrade the SlideX agent server to the public Heddle 5.1.0 packages
• add a user-scoped Supabase implementation of Heddle's append-only chat archive repository
• compose the session and archive repositories through one shared server-only Supabase client
• document the storage boundary, migration dependency, cutover behavior, and durability acceptance check

Dependency and rollout order

This PR depends on the editor-owned schema in roackb2/SlideX#24.

1. Merge and deploy roackb2/SlideX#24 to the target Supabase project.
2. Confirm the archive tables and append_agent_session_archive RPC exist.
3. Deploy this server branch.
4. Enable HEDDLE_SESSION_STORAGE=supabase only after the schema is live.

No manual table design is required. The migration is the authoritative schema.

Verification

• npm ci against the public npm registry
• npm test (80 tests passed)
• npm run typecheck
• npm run build
• real local Supabase acceptance: append through the RPC, load manifest and summary from a fresh Node process, hide data across users, reject duplicates, and deny direct archive updates with PostgreSQL 42501

Remaining rollout acceptance

After the migration is deployed in the target project, force one real Heddle compaction, replace the server process, and confirm the next turn restores the manifest and rolling summary. This is a deployment acceptance check, not an adapter-code dependency.