feat: GPL backend self-provisions example libs + standalone port override

Port the KiCad symbol/footprint brace-scanner parsers + a combined extractor
into web/backend/src/extract/; ensure-example-libs blobless/sparse-clones a
curated KiCad 10.0.3 slice and extracts to a gitignored .libs/ on dev/start, so
a bare GPL clone has libraries with no closed repo present. libsConfig() defaults
LIBS_DIR to ./.libs. vite honors STANDALONE_PORT (strictPort) so a second editor
can run on :3049 alongside the closed stack.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/backend/package.json

@@ -4,8 +4,10 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "dev": "tsx watch src/server.ts",
-    "start": "tsx src/server.ts",
+    "dev": "tsx src/extract/ensure-example-libs.ts && tsx watch src/server.ts",
+    "start": "tsx src/extract/ensure-example-libs.ts && tsx src/server.ts",
+    "ensure-libs": "tsx src/extract/ensure-example-libs.ts",
+    "extract-libs": "tsx src/extract/extract-libs.ts",
     "build": "tsc -p tsconfig.json --noEmit false --declaration false --outDir dist",
     "typecheck": "tsc --noEmit"
   },