fix: add date_posted migration + cloud startup sweep

date_posted column was added to db.py CREATE TABLE but had no migration
file, so existing user DBs were missing it. The list_jobs endpoint queries
this column, causing 500 errors and empty Apply/Review queues for all
existing cloud users while job_counts (which doesn't touch date_posted)
continued to work — making the home page show correct counts but tabs show
empty data.

Fixes:
- migrations/006_date_posted.sql: ALTER TABLE to add date_posted to existing DBs
- dev_api.py lifespan: on startup in cloud mode, sweep all user DBs in
  CLOUD_DATA_ROOT and apply pending migrations — ensures schema changes land
  for every user on each deploy, not only on their first post-deploy request

migrations/006_date_posted.sql

@@ -0,0 +1,6 @@
+-- 006_date_posted.sql
+-- Add date_posted column for shadow listing detection (stale/shadow score feature).
+-- New DBs already have this column from the CREATE TABLE statement in db.py;
+-- this migration adds it to existing user DBs.
+
+ALTER TABLE jobs ADD COLUMN date_posted TEXT;