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/.env.example

@@ -1,4 +1,9 @@
 # Thin reference backend. Copy to backend/.env.
+#
+# It uses NO database — it serves a single project folder off disk plus a small
+# set of example libraries. The example libs are self-provisioned on `dev`/`start`
+# (see src/extract/ensure-example-libs.ts): a curated slice of upstream KiCad
+# symbol + footprint libs is cloned + extracted into ./.libs the first time.
 
 # Absolute or relative path to the single KiCad project folder to serve.
 PROJECT_DIR=../../tests/fixtures/demo
@@ -8,3 +13,11 @@ PORT=3060
 
 # Browser origin allowed to call this backend (the Vite dev server).
 CORS_ORIGIN=http://localhost:3048
+
+# Origin (read-only) libraries dir. Unset ⇒ defaults to ./.libs (auto-provisioned).
+# Point it at your own fixtures tree to override.
+# LIBS_DIR=./.libs
+
+# Writable user libraries dir (symbols/footprints created in the editor).
+# Unset ⇒ defaults to ./.user-libs.
+# USER_LIBS_DIR=./.user-libs