[Data Isolation][108dev] Questions assemblies endpoint includes essay assemblies

## Summary
After restoring Questions API, `/api/questions/assemblies` returns essay assembly records (e.g., `asm-essay-*`) in the evaluator assembly list.

## Impact
- Questions UI may show essay evaluator assemblies.
- Users can pick incompatible assemblies, producing evaluation inconsistencies.

## Evidence
Sample records observed from `/api/questions/assemblies`:
- `asm-essay-fund1-001`
- `asm-essay-em-001`
- `asm-essay-econ-001`

## Suspected root cause
Questions and Essays appear to read/write from a shared `assemblies` container without domain discriminator filtering.

## Immediate mitigation idea
- Filter questions assembly queries to include only question-domain assemblies.
- Add domain/type discriminator on assembly documents (`domain=questions|essays`) and enforce per-service filters.

## Longer-term
- Backport to IaC/data model and add regression tests for domain isolation.

Related: #139, #141, #144