feat(backend): migrate production backend to Rust

Replace the Go backend with the Rust workspace and route Android/iOS through
UniFFI bindings. Include the migrated extension runtime, network, providers,
media metadata, downloads and library operations, with version 5.0.0+147.

Remove Go sources, adapters, native build selection and CI dependencies.
Build Rust unconditionally and lock the iOS Rust pod using a relative path.
Retain legacy source and migration evidence in a local ignored archive.

Validation: Android Kotlin compile and 53 native tests; Swift Rust-branch
equivalence and syntax; CocoaPods install; workflow, shell, Ruby, plist and
diff checks. Reuse the preceding 100 Rust tests, fmt/Clippy and five-ABI
build checkpoint; no new APK or iOS application build for this cleanup.

Known follow-up: URL/URLSearchParams globals are missing from the Rust JS
runtime. A controlled extension replay confirms a URL-resolution regression;
this commit does not fix that runtime gap.

.github/PULL_REQUEST_TEMPLATE.md

@@ -22,7 +22,7 @@
 - [ ] `dart format --output=none --set-exit-if-changed lib test`
 - [ ] `flutter analyze`
 - [ ] `flutter test`
-- [ ] `gofmt`, `go vet ./...`, and `go test ./...` in `go_backend/`
+- [ ] `bash scripts/check_rust_backend.sh`
 - [ ] Android or iOS native checks
 - [ ] Manual reproduction or device test