Add database overview feature with metrics and UI enhancements

This commit introduces a detailed database overview endpoint and service, providing key metrics such as replication status, database sizes, and performance indicators. On the frontend, a new UI section displays these metrics along with improved forms and troubleshooting tips. Enhancements improve user experience by adding informative tooltips and formatting for byte and time values.

backend/app/services/infra_probe.py

@@ -0,0 +1,29 @@
+from dataclasses import dataclass
+
+
+@dataclass
+class DiskSpaceProbeResult:
+    source: str
+    status: str
+    free_bytes: int | None = None
+    message: str | None = None
+
+
+class DiskSpaceProvider:
+    async def get_free_bytes(self, target_host: str) -> DiskSpaceProbeResult:
+        raise NotImplementedError
+
+
+class NullDiskSpaceProvider(DiskSpaceProvider):
+    async def get_free_bytes(self, target_host: str) -> DiskSpaceProbeResult:
+        return DiskSpaceProbeResult(
+            source="none",
+            status="unavailable",
+            free_bytes=None,
+            message=f"No infra probe configured for host {target_host}. Add SSH/Agent provider later.",
+        )
+
+
+def get_disk_space_provider() -> DiskSpaceProvider:
+    # Extension point: replace by SSH/Agent backed implementation.
+    return NullDiskSpaceProvider()