feat: track imported accounts per upstream key across platforms

Add upstream_key_account_links table mapping generated keys to remote
accounts per website/platform, surface imported_accounts on key
responses, and update website sync/routers to manage the links.

backend/app/schemas/upstream.py

@@ -134,6 +134,20 @@ class GenerateKeysByGroupsRequest(BaseModel):
     endpoint: str = "/keys"
 
 
+class UpstreamKeyAccountLinkResponse(BaseModel):
+    id: int
+    upstream_key_id: int
+    website_id: int
+    target_group: str
+    platform: str
+    remote_account_id: str
+    status: str
+    created_at: Optional[datetime] = None
+    updated_at: Optional[datetime] = None
+
+    model_config = {"from_attributes": True}
+
+
 class GeneratedUpstreamKeyResponse(BaseModel):
     id: Optional[int] = None
     upstream_id: int
@@ -151,6 +165,7 @@ class GeneratedUpstreamKeyResponse(BaseModel):
     created_at: Optional[datetime] = None
     updated_at: Optional[datetime] = None
     has_key_value: bool = False
+    imported_accounts: list[UpstreamKeyAccountLinkResponse] = Field(default_factory=list)
 
 
 class GenerateKeysByGroupsResponse(BaseModel):