Perf: light worker poll query + tasks(status) index

Worker polls the DB every interval (production 30s) and every
poll ran SELECT * FROM tasks: full rows including result_text
and error_text (up to 4000 chars each) for every task ever,
growing with history. load_tasks now selects only the columns
pick_next actually needs (id/project/prompt/status/depends/
priority/timestamps/executor) and only queued/completed/merged
rows; running/failed rows are neither ready nor completed, so
excluding them cannot change pick_next. get_task still does
SELECT * for the single claimed task. Plus CREATE INDEX IF NOT
EXISTS idx_tasks_status on tasks(status) in both SQLite
init_db and the Postgres schema, so the poll filter stops being
a full table scan on old databases too. Test: load_tasks returns
only queued+completed out of queued/completed/running/failed.

cloud/db.py

@@ -807,6 +807,7 @@ POSTGRES_SCHEMA = (
     "pr_url TEXT, "
     "pr_number INTEGER, "
     "UNIQUE(project_id, idempotency_key))",
+    "CREATE INDEX IF NOT EXISTS idx_tasks_status ON tasks(status)",
     "CREATE TABLE IF NOT EXISTS summaries("
     "id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY, "
     "project_id TEXT, "