fix: postgres WAL corruption recovery + memory bump + researcher/executor

- Postgres memory: 256M→384M limits, 64M→96M reservations
- Added pg_resetwal -f pre-deploy step to recover from corrupt WAL
  ('PANIC: could not locate a valid checkpoint record' caused by
  force-killed postgres during --force-recreate)
- Added data-checksums initdb arg for future corruption detection
- api→postgres and web→api depends_on: service_healthy→service_started
- Deploy wait loop: fail fast on unhealthy, wait on starting (180s)
- Added researcher/executor to ValidAssignees and frontend dropdowns

compose.yaml

@@ -7,10 +7,11 @@ services:
     deploy:
       resources:
         limits:
-          memory: 256M
+          memory: 384M
         reservations:
-          memory: 64M
+          memory: 96M
     environment:
+      POSTGRES_INITDB_ARGS: --data-checksums
       POSTGRES_DB: ${POSTGRES_DB:-nexus}
       POSTGRES_USER: ${POSTGRES_USER:-nexus}
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?Set POSTGRES_PASSWORD in .env}