feat(FN-1769): add pluggable memory backend system with QMD support

- Add MemoryBackend interface with file and QMD backend implementations
- Implement QMD memory backend with async execution, atomic writes, and fallback to file backend
- Add ProjectMemory class with backend-aware read/bootstrap semantics and conflict resolution
- Wire dashboard memory routes to use backend abstraction instead of direct file I/O
- Add comprehensive tests for memory backend and project memory classes
- Update settings reference and architecture docs for new backend configuration

docs/architecture.md

@@ -195,7 +195,7 @@ const backend = resolveMemoryBackend(settings);
 
 **Settings integration:**
 - `memoryEnabled`: Toggle controls whether memory instructions are injected into prompts
-- `memoryBackendType`: Select which backend to use (`file` or `readonly`)
+- `memoryBackendType`: Select which backend to use (`file`, `readonly`, `qmd`, or custom). Unknown types are accepted and persisted verbatim; the system falls back to `file` at runtime.
 
 **Dashboard API:**
 - `GET /api/memory/backend` — Returns current backend status and capabilities