refactor: govern background job execution

backend/database/migrations/m0002_job_runs.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+import sqlite3
+
+from backend.database.migrations.runner import Migration
+
+
+def create_job_runs(connection: sqlite3.Connection) -> None:
+    connection.execute(
+        """
+        CREATE TABLE IF NOT EXISTS job_runs (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            job_id TEXT NOT NULL,
+            idempotency_key TEXT NOT NULL,
+            status TEXT NOT NULL,
+            attempt INTEGER NOT NULL DEFAULT 1,
+            started_at TEXT NOT NULL,
+            finished_at TEXT,
+            elapsed_ms INTEGER NOT NULL DEFAULT 0,
+            error_code TEXT NOT NULL DEFAULT '',
+            message TEXT NOT NULL DEFAULT '',
+            output_version TEXT NOT NULL DEFAULT '',
+            metadata TEXT NOT NULL DEFAULT '{}',
+            UNIQUE(job_id, idempotency_key, attempt)
+        )
+        """
+    )
+    connection.execute(
+        """
+        CREATE INDEX IF NOT EXISTS idx_job_runs_job_started
+        ON job_runs(job_id, started_at DESC, id DESC)
+        """
+    )
+    connection.execute(
+        """
+        CREATE INDEX IF NOT EXISTS idx_job_runs_status
+        ON job_runs(status, started_at DESC, id DESC)
+        """
+    )
+
+
+MIGRATION = Migration(
+    version="0002",
+    name="create_job_runs",
+    action=create_job_runs,
+    signature="job-runs:v1:id,job,key,status,attempt,times,elapsed,error,output,metadata",
+)