Fix canned_responses 500: add missing SQL migration

The canned_responses table was registered in autoMigrate() (app.go) but
autoMigrate() is never called on the serve path (Bootstrap -> NewDatabase
-> RunMigrations only). With no SQL migration file, the table never
existed, so POST /api/v1/accounts/:id/canned_responses failed with
'relation does not exist' -> 500.

Add migration 000049 to create the table matching the GORM model
(id, account_id, content, short_code, timestamps) with the account+
short_code partial unique index and deleted_at index.

Also include two related fixes staged in the working tree:
- router.go: register notifications.GET("") alongside GET("/") to avoid
  Gin 301 trailing-slash redirect that the Vite proxy doesn't follow
- notifications/actions.js: stop the infinite loader on fetch error to
  prevent IntersectionObserver re-fire loop

backend/migrations/000049_add_canned_responses_table.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS canned_responses;

backend/migrations/000049_add_canned_responses_table.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS canned_responses (
+    id SERIAL PRIMARY KEY,
+    account_id INTEGER NOT NULL,
+    content TEXT NOT NULL,
+    short_code VARCHAR(255) NOT NULL,
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW(),
+    deleted_at TIMESTAMPTZ
+);
+CREATE INDEX IF NOT EXISTS idx_canned_responses_account_id ON canned_responses(account_id);
+CREATE UNIQUE INDEX IF NOT EXISTS idx_canned_short_code ON canned_responses(account_id, short_code) WHERE deleted_at IS NULL;
+CREATE INDEX IF NOT EXISTS idx_canned_responses_deleted_at ON canned_responses(deleted_at);