feat: Telegram settings in DB with UI edit and test send

Store notification_channels (migration 004), effective config DB over env,
PUT/test API endpoints, Settings form, and pass db session into ingest notify.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/__init__.py

@@ -1,6 +1,7 @@
 from app.models.api_key import ApiKey
 from app.models.event import Event
 from app.models.host import Host
+from app.models.notification_channel import NotificationChannel
 from app.models.problem import Problem, ProblemEvent
 
-__all__ = ["ApiKey", "Event", "Host", "Problem", "ProblemEvent"]
+__all__ = ["ApiKey", "Event", "Host", "NotificationChannel", "Problem", "ProblemEvent"]

backend/app/models/notification_channel.py

@@ -0,0 +1,19 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, String, Text, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+
+class NotificationChannel(Base):
+    __tablename__ = "notification_channels"
+
+    channel: Mapped[str] = mapped_column(String(32), primary_key=True)
+    enabled: Mapped[bool] = mapped_column(default=False)
+    min_severity: Mapped[str] = mapped_column(String(16), default="warning")
+    bot_token: Mapped[str | None] = mapped_column(Text, nullable=True)
+    chat_id: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )