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/models/response.py

@@ -183,7 +183,7 @@ class PaginationMetadata(BanGuiBaseModel):
         description="Opaque cursor token for fetching the next page (cursor pagination only).",
     )
     pagination_mode: Literal["offset", "cursor"] = Field(
-        ...,
+        default="offset",
         description="Pagination mode used by the endpoint. 'offset' uses page/page_size; 'cursor' uses cursor tokens.",
     )
 
@@ -353,6 +353,7 @@ class ErrorMetadata(TypedDict, total=False):
         filter_name: Name of the filter involved in the error.
         action_name: Name of the action involved in the error.
         source_id: ID of a blocklist source involved in the error.
+        url: URL involved in a blocklist error.
         ip: IP address involved in the error.
         pattern: Regex pattern that caused an error.
         error: Regex compilation error message.
@@ -371,6 +372,7 @@ class ErrorMetadata(TypedDict, total=False):
     filter_name: str
     action_name: str
     source_id: int
+    url: str
     ip: str
     pattern: str
     error: str
@@ -412,6 +414,7 @@ class HealthResponse(BanGuiBaseModel):
         database: Database connectivity — 'ok' or 'error'.
         scheduler: Background scheduler status — 'running', 'stopped', or 'unknown'.
         cache: Cache initialization status — 'initialised' or 'uninitialised'.
+        external_logging: External logging handler status — 'ok', 'error', or 'disabled'.
         components: Per-component health detail list (empty when all healthy).
 
     Example:
@@ -423,6 +426,7 @@ class HealthResponse(BanGuiBaseModel):
           "database": "ok",
           "scheduler": "running",
           "cache": "initialised",
+          "external_logging": "disabled",
           "components": []
         }
 
@@ -433,6 +437,7 @@ class HealthResponse(BanGuiBaseModel):
           "database": "ok",
           "scheduler": "running",
           "cache": "initialised",
+          "external_logging": "ok",
           "components": [{"name": "fail2ban", "healthy": false, "message": "Socket not reachable"}]
         }
         ```
@@ -461,6 +466,13 @@ class HealthResponse(BanGuiBaseModel):
         ...,
         description="Cache initialization status: 'initialised' when ready, 'uninitialised' when not.",
     )
+    external_logging: Literal["ok", "error", "disabled"] = Field(
+        ...,
+        description=(
+            "External logging handler status: 'ok' when operational, 'error' when "
+            "initialization failed, 'disabled' when external logging is not configured."
+        ),
+    )
     components: list[ComponentHealth] = Field(
         default_factory=list,
         description="Per-component health detail list. Empty when status is 'ok'.",