Round-trip `letterId` through backend Capture snapshot

[wondopamine]

Context

Follow-up surfaced by CE code review of #26. The Letters → Capture flow in that PR stamps a letterId onto every ask capture so AskSheet can surface a clickable "From — " pill linking back to the originating letter. The pill works on the local-only path (write → persist → reload from ss:v1:*), but is silently dropped on the backend round-trip.

Symptom

After a full session + backend snapshot reload (State.applyBackendSnapshot → Captures.upsertBackend), the ask capture loses its letterId because StudentSpaceReflectionCaptureSnapshot doesn't carry the field. The pill disappears from replay; the capture text + reframe + thread are intact.

Layered fix

Layer	Change
DB migration	Add nullable letter_id text column to mirror_entries (see src/db/schema.ts:65)
Queries	MirrorEntryRow type in src/db/queries.ts auto-picks up the column; verify select lists include it
Write path	src/server/run-mirror.handler.server.ts (and any other path inserting into mirror_entries) forwards letterId from the request body
Read path	mapShellMirrorEntries at src/lib/student-space/backend-snapshot.ts:266 forwards entry.letterId into the snapshot
Type	Add letterId: string | null to StudentSpaceReflectionCaptureSnapshot
Engine schema	Relax the mergeCapture typeof === 'string' guard for letterId to allow null (so backend-cleared values don't trip the validator)

Why this wasn't in #26

The fix crosses a DB migration boundary and the user-visible failure mode is narrow (clickable back-reference only — capture data is intact). Bundling it would mix UI work with schema work and inflate review surface. Better as its own PR with focused migration review.

Related

• PR Split-layout sheets (U1-U8) + teacher-prompt Capture flow #26 — Split-layout sheets + teacher-prompt Capture flow
• CE artifact: .context/compound-engineering/ce-code-review/20260521-142136-d51fbba5/correctness.json