feat(notifications): persistent notification center

- notifications table (goose migration) + sqlc queries; list with unread
  count, mark-one, mark-all endpoints under /notifications.
- Deployment success/failure now writes a notification row for the owner
  (insertUserNotification — best-effort, never breaks the deploy flow).
- Shell bell reads the real persisted feed: unread badge, click-to-read,
  mark-all, 30s polling. Replaces the transient builds+audit derivation.

Verified e2e: migration applies clean, list/read/read-all round-trips.

Authored By: TDvorak <[email protected]>

app/backend/migrations_goose/20260920000000_notifications.sql

@@ -0,0 +1,19 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS public.notifications (
+    id uuid DEFAULT gen_random_uuid() NOT NULL,
+    user_id uuid NOT NULL,
+    kind character varying(50) NOT NULL,
+    title character varying(255) NOT NULL,
+    body text,
+    resource_type character varying(50),
+    resource_id character varying(255),
+    read_at timestamp with time zone,
+    created_at timestamp with time zone DEFAULT now(),
+    CONSTRAINT notifications_pkey PRIMARY KEY (id),
+    CONSTRAINT fk_notifications_user FOREIGN KEY (user_id) REFERENCES public.users(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_notifications_user_created ON public.notifications(user_id, created_at DESC);
+
+-- +goose Down
+DROP TABLE IF EXISTS public.notifications;