feat: webhook notification channel with UI and ingest dispatch

Add webhook config (DB/env), JSON POST on events/problems, settings API,
Settings UI section, and notify_dispatch for multi-channel ingest.

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

backend/alembic/versions/005_notification_webhook.py

@@ -0,0 +1,30 @@
+"""notification_channels: webhook columns
+
+Revision ID: 005
+Revises: 004
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "005"
+down_revision: Union[str, None] = "004"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("notification_channels", sa.Column("webhook_url", sa.Text(), nullable=True))
+    op.add_column(
+        "notification_channels",
+        sa.Column("webhook_secret_header", sa.String(length=64), nullable=True),
+    )
+    op.add_column("notification_channels", sa.Column("webhook_secret", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("notification_channels", "webhook_secret")
+    op.drop_column("notification_channels", "webhook_secret_header")
+    op.drop_column("notification_channels", "webhook_url")