add tagging system for media items

Introduces user-defined tag categories and tags with a many-to-many
relationship to media items. Tags are stored in a SQLite database
(medialore.db via better-sqlite3) with ON DELETE CASCADE for automatic
cleanup. Users can manage categories and tags at /manage/tags, assign
tags to games in the detail modal, and tag mixed media files via a
hover button on each tile.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/api/libraries/[id]/route.ts

@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { getLibrary, removeLibrary } from '@/lib/libraries'
+import { removeAllAssignmentsForLibrary } from '@/lib/tags'
 
 export async function DELETE(
   _request: NextRequest,
@@ -13,5 +14,6 @@ export async function DELETE(
   }
 
   removeLibrary(id)
+  removeAllAssignmentsForLibrary(id)
   return new NextResponse(null, { status: 204 })
 }