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/schemas/overview.py

@@ -0,0 +1,79 @@
+from datetime import datetime
+from pydantic import BaseModel
+
+
+class DbSizeItem(BaseModel):
+    name: str
+    size_bytes: int
+
+
+class TableSizeItem(BaseModel):
+    schema: str
+    table: str
+    size_bytes: int
+
+
+class ReplicationClientItem(BaseModel):
+    application_name: str | None = None
+    client_addr: str | None = None
+    state: str | None = None
+    sync_state: str | None = None
+    write_lag_seconds: float | None = None
+    flush_lag_seconds: float | None = None
+    replay_lag_seconds: float | None = None
+    replay_lag_bytes: float | None = None
+
+
+class DiskSpaceInfo(BaseModel):
+    source: str
+    status: str
+    free_bytes: int | None = None
+    message: str | None = None
+
+
+class InstanceOverview(BaseModel):
+    server_version: str | None = None
+    role: str
+    postmaster_start_time: datetime | None = None
+    uptime_seconds: float | None = None
+    current_database: str | None = None
+    port: int | None = None
+
+
+class StorageOverview(BaseModel):
+    current_database_size_bytes: int | None = None
+    all_databases: list[DbSizeItem]
+    largest_tables: list[TableSizeItem]
+    wal_directory_size_bytes: int | None = None
+    disk_space: DiskSpaceInfo
+
+
+class ReplicationOverview(BaseModel):
+    mode: str
+    replay_lag_seconds: float | None = None
+    replication_slots_count: int | None = None
+    active_replication_clients: int | None = None
+    clients: list[ReplicationClientItem]
+
+
+class PerformanceOverview(BaseModel):
+    xact_commit: int | None = None
+    xact_rollback: int | None = None
+    deadlocks: int | None = None
+    temp_files: int | None = None
+    temp_bytes: int | None = None
+    blk_read_time: float | None = None
+    blk_write_time: float | None = None
+    checkpoints_timed: int | None = None
+    checkpoints_req: int | None = None
+    autovacuum_workers: int | None = None
+
+
+class DatabaseOverviewOut(BaseModel):
+    collected_at: datetime
+    cache_ttl_seconds: int
+    instance: InstanceOverview
+    storage: StorageOverview
+    replication: ReplicationOverview
+    performance: PerformanceOverview
+    partial_failures: list[str]