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/alembic/versions/004_notification_channels.py

@@ -0,0 +1,37 @@
+"""notification_channels for UI-managed Telegram settings
+
+Revision ID: 004
+Revises: 003
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "004"
+down_revision: Union[str, None] = "003"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "notification_channels",
+        sa.Column("channel", sa.String(length=32), nullable=False),
+        sa.Column("enabled", sa.Boolean(), nullable=False, server_default=sa.text("false")),
+        sa.Column("min_severity", sa.String(length=16), nullable=False, server_default="warning"),
+        sa.Column("bot_token", sa.Text(), nullable=True),
+        sa.Column("chat_id", sa.String(length=64), nullable=True),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            nullable=False,
+        ),
+        sa.PrimaryKeyConstraint("channel"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("notification_channels")