feat: add GitHub activity sync background job

Adds automatic syncing of repository activity for projects with github_repo set. Background task fetches latest activity from GitHub API (issues, PRs, default branch commits) and updates last_github_activity timestamp. Configurable sync interval (default: 15 minutes), requires GITHUB_TOKEN env var.

.sqlx/query-82118a82eb002e4155689f37c50b7a286d2381d9a195f1665979d3a338bedc01.json

@@ -0,0 +1,15 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "UPDATE projects SET last_github_activity = $2 WHERE id = $1",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Left": [
+        "Uuid",
+        "Timestamptz"
+      ]
+    },
+    "nullable": []
+  },
+  "hash": "82118a82eb002e4155689f37c50b7a286d2381d9a195f1665979d3a338bedc01"
+}