fix(migrations): add 0004 for search_providers table

Add incremental migration for existing databases to create the
search_providers table and bots.search_provider_id column introduced
in the search provider feature.

db/migrations/0004_search_providers.down.sql

@@ -0,0 +1,3 @@
+-- 0005_search_providers (down)
+ALTER TABLE bots DROP COLUMN IF EXISTS search_provider_id;
+DROP TABLE IF EXISTS search_providers;

db/migrations/0004_search_providers.up.sql

@@ -0,0 +1,15 @@
+-- 0005_search_providers
+-- Add search_providers table and link to bots for web search integration.
+
+CREATE TABLE IF NOT EXISTS search_providers (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  name TEXT NOT NULL,
+  provider TEXT NOT NULL,
+  config JSONB NOT NULL DEFAULT '{}'::jsonb,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  CONSTRAINT search_providers_name_unique UNIQUE (name),
+  CONSTRAINT search_providers_provider_check CHECK (provider IN ('brave'))
+);
+
+ALTER TABLE bots ADD COLUMN IF NOT EXISTS search_provider_id UUID REFERENCES search_providers(id) ON DELETE SET NULL;