feat(db): Add favicon support and improve remote followers uniqueness constraint

- Add favicon_url column to nodes table for storing node instance favicons
- Replace actor_url unique constraint with composite unique index on (user_id, actor_url) in remote_followers table
- Update database schema and migration files to reflect new constraints
- Improve data integrity by ensuring remote followers are unique per user and actor combination

drizzle/0005_yummy_millenium_guard.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "remote_followers" DROP CONSTRAINT "remote_followers_actor_url_unique";--> statement-breakpoint
+ALTER TABLE "nodes" ADD COLUMN "favicon_url" text;--> statement-breakpoint
+CREATE UNIQUE INDEX "remote_followers_user_actor_unique" ON "remote_followers" USING btree ("user_id","actor_url");