feat: comprehensive health check with DB, scheduler, cache

- Add /api/v1/health endpoint with component-level checks
- Verify DB connectivity, fail2ban socket, scheduler, session cache
- Add SQLite WAL cleanup on startup (orphan crash files)
- Migration 8: import_log.timestamp → INTEGER UNIX epoch
- Align import_log timestamps with history_archive (already UNIX int)
- Add unit tests for DB cleanup and health router

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

backend/app/models/blocklist.py

@@ -64,7 +64,7 @@ class ImportLogEntry(BanGuiBaseModel):
     id: int
     source_id: int | None
     source_url: str
-    timestamp: str
+    timestamp: int
     ips_imported: int
     ips_skipped: int
     errors: str | None

backend/app/models/response.py

@@ -328,37 +328,87 @@ class ErrorResponse(BanGuiBaseModel):
     )
 
 
+class ComponentHealth(BanGuiBaseModel):
+    """Health status of a single application component.
+
+    Fields:
+        name: Human-readable component name.
+        healthy: True when the component is operational.
+        message: Optional detail message (e.g., error description).
+    """
+
+    name: str = Field(..., description="Component name.")
+    healthy: bool = Field(..., description="True when the component is operational.")
+    message: str | None = Field(
+        default=None,
+        description="Optional detail message, e.g. error description.",
+    )
+
+
 class HealthResponse(BanGuiBaseModel):
     """Standardized response for the health check endpoint.
 
     Fields:
-        status: Application health status — 'ok' when healthy, 'unavailable' otherwise.
+        status: Application health status — 'ok' when all components are healthy,
+            'degraded' when some components are unhealthy but the service can still
+            handle requests, 'unavailable' when fail2ban is offline.
         fail2ban: fail2ban daemon status — 'online' or 'offline'.
+        database: Database connectivity — 'ok' or 'error'.
+        scheduler: Background scheduler status — 'running', 'stopped', or 'unknown'.
+        cache: Cache initialization status — 'initialised' or 'uninitialised'.
+        components: Per-component health detail list (empty when all healthy).
 
     Example:
         ```python
         # Healthy (HTTP 200)
         {
           "status": "ok",
-          "fail2ban": "online"
+          "fail2ban": "online",
+          "database": "ok",
+          "scheduler": "running",
+          "cache": "initialised",
+          "components": []
         }
 
         # Unhealthy (HTTP 503)
         {
           "status": "unavailable",
-          "fail2ban": "offline"
+          "fail2ban": "offline",
+          "database": "ok",
+          "scheduler": "running",
+          "cache": "initialised",
+          "components": [{"name": "fail2ban", "healthy": false, "message": "Socket not reachable"}]
         }
         ```
     """
 
-    status: Literal["ok", "unavailable"] = Field(
+    status: Literal["ok", "degraded", "unavailable"] = Field(
         ...,
-        description="Application health status: 'ok' when healthy, 'unavailable' otherwise.",
+        description=(
+            "Application health status: 'ok' when healthy, 'degraded' when some "
+            "components are unhealthy, 'unavailable' when fail2ban is offline."
+        ),
     )
     fail2ban: Literal["online", "offline"] = Field(
         ...,
         description="fail2ban daemon status: 'online' when reachable, 'offline' otherwise.",
     )
+    database: Literal["ok", "error"] = Field(
+        ...,
+        description="Database connectivity: 'ok' when accessible, 'error' when not.",
+    )
+    scheduler: Literal["running", "stopped", "unknown"] = Field(
+        ...,
+        description="Background scheduler status: 'running', 'stopped', or 'unknown'.",
+    )
+    cache: Literal["initialised", "uninitialised"] = Field(
+        ...,
+        description="Cache initialization status: 'initialised' when ready, 'uninitialised' when not.",
+    )
+    components: list[ComponentHealth] = Field(
+        default_factory=list,
+        description="Per-component health detail list. Empty when status is 'ok'.",
+    )
 
 
 class FlushLogsResponse(BanGuiBaseModel):