Add test connection feature for database targets

This commit introduces a new endpoint to test database connection. The frontend now includes a button to test the connection before creating a target, with real-time feedback on success or failure. Related styles and components were updated for better user experience.

backend/app/schemas/target.py

@@ -16,6 +16,15 @@ class TargetCreate(TargetBase):
     password: str
 
 
+class TargetConnectionTestRequest(BaseModel):
+    host: str
+    port: int = 5432
+    dbname: str
+    username: str
+    password: str
+    sslmode: str = "prefer"
+
+
 class TargetUpdate(BaseModel):
     name: str | None = None
     host: str | None = None