Add target owners and alert notification management.

This commit implements the addition of `target_owners` and `alert_notification_events` tables, enabling management of responsible users for targets. Backend and frontend components are updated to allow viewing, assigning, and notifying target owners about critical alerts via email.

backend/app/schemas/target.py

@@ -10,6 +10,7 @@ class TargetBase(BaseModel):
     username: str
     sslmode: str = "prefer"
     use_pg_stat_statements: bool = True
+    owner_user_ids: list[int] = Field(default_factory=list)
     tags: dict = Field(default_factory=dict)
 
 
@@ -35,6 +36,7 @@ class TargetUpdate(BaseModel):
     password: str | None = None
     sslmode: str | None = None
     use_pg_stat_statements: bool | None = None
+    owner_user_ids: list[int] | None = None
     tags: dict | None = None
 
 
@@ -43,3 +45,13 @@ class TargetOut(TargetBase):
     created_at: datetime
 
     model_config = {"from_attributes": True}
+
+
+class TargetOwnerOut(BaseModel):
+    user_id: int
+    email: str
+    role: str
+
+
+class TargetOwnersUpdate(BaseModel):
+    user_ids: list[int] = Field(default_factory=list)