feat(06-01): add config history service with TDD tests

- Service queries router_config_changes JOIN router_config_diffs for timeline
- Returns paginated entries with component, summary, timestamp, diff metadata
- ORDER BY created_at DESC with limit/offset pagination
- 4 tests covering formatting, empty results, pagination, and ordering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/config_history_service.py

@@ -0,0 +1,64 @@
+"""Config history timeline service.
+
+Provides paginated query of config change entries for a device,
+joining router_config_changes with router_config_diffs to include
+diff metadata (lines_added, lines_removed, snapshot_id).
+"""
+
+import logging
+
+from sqlalchemy import text
+
+logger = logging.getLogger(__name__)
+
+
+async def get_config_history(
+    device_id: str,
+    tenant_id: str,
+    session,
+    limit: int = 50,
+    offset: int = 0,
+) -> list[dict]:
+    """Return paginated config change timeline for a device.
+
+    Joins router_config_changes with router_config_diffs to get
+    diff metadata alongside each change entry. Results are ordered
+    by created_at DESC (newest first).
+
+    Returns a list of dicts with: id, component, summary, created_at,
+    diff_id, lines_added, lines_removed, snapshot_id.
+    """
+    result = await session.execute(
+        text(
+            "SELECT c.id, c.component, c.summary, c.created_at, "
+            "d.id AS diff_id, d.lines_added, d.lines_removed, "
+            "d.new_snapshot_id AS snapshot_id "
+            "FROM router_config_changes c "
+            "JOIN router_config_diffs d ON c.diff_id = d.id "
+            "WHERE c.device_id = CAST(:device_id AS uuid) "
+            "AND c.tenant_id = CAST(:tenant_id AS uuid) "
+            "ORDER BY c.created_at DESC "
+            "LIMIT :limit OFFSET :offset"
+        ),
+        {
+            "device_id": device_id,
+            "tenant_id": tenant_id,
+            "limit": limit,
+            "offset": offset,
+        },
+    )
+    rows = result.fetchall()
+
+    return [
+        {
+            "id": str(row._mapping["id"]),
+            "component": row._mapping["component"],
+            "summary": row._mapping["summary"],
+            "created_at": row._mapping["created_at"].isoformat(),
+            "diff_id": str(row._mapping["diff_id"]),
+            "lines_added": row._mapping["lines_added"],
+            "lines_removed": row._mapping["lines_removed"],
+            "snapshot_id": str(row._mapping["snapshot_id"]),
+        }
+        for row in rows
+    ]