refactor(backend): external logging metrics, required mode, health checks

- Add external_logging_init_failures counter
- Add external_log_required flag, raise if init fails and required
- Health endpoint: add external_logging status check
- Blocklist service: enrich with metadata fields, update import logic
- Health check task: add runtime_state dependency, fix return typing
- Metrics: add Histogram for request latencies
- Frontend: align BlocklistImportLogSection props
- Docs: update deployment guide, remove stale tasks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/repositories/blocklist_repo.py

@@ -13,6 +13,37 @@ if TYPE_CHECKING:
     import aiosqlite
 
 
+async def create_source_in_tx(
+    db: aiosqlite.Connection,
+    name: str,
+    url: str,
+    *,
+    enabled: bool = True,
+) -> int:
+    """Insert a new blocklist source without committing.
+
+    Caller is responsible for committing or rolling back the transaction.
+    Use this variant when validation must be atomic with insert.
+
+    Args:
+        db: Active aiosqlite connection with an open transaction.
+        name: Human-readable display name.
+        url: URL of the blocklist text file.
+        enabled: Whether the source is active.  Defaults to ``True``.
+
+    Returns:
+        The ``ROWID`` / primary key of the new row.
+    """
+    cursor = await db.execute(
+        """
+        INSERT INTO blocklist_sources (name, url, enabled)
+        VALUES (?, ?, ?)
+        """,
+        (name, url, int(enabled)),
+    )
+    return int(cursor.lastrowid)  # type: ignore[arg-type]
+
+
 async def create_source(
     db: aiosqlite.Connection,
     name: str,