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.

scripts/build_ios.sh

@@ -1,45 +1,7 @@
 #!/usr/bin/env bash
-# Build the Go backend XCFramework used by SpotiFLAC Mobile on iOS.
+# Build the Rust backend XCFramework used by SpotiFLAC Mobile on iOS.
 
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-PROJECT_DIR="$(dirname "$SCRIPT_DIR")"
-GO_BACKEND_DIR="$PROJECT_DIR/go_backend"
-OUTPUT_DIR="$PROJECT_DIR/ios/Frameworks"
-
-echo "SpotiFLAC Mobile iOS backend build"
-echo "Project: $PROJECT_DIR"
-echo "Output:  $OUTPUT_DIR"
-
-if [[ "$(uname)" != "Darwin" ]]; then
-  echo "Error: this script must run on macOS with Xcode installed." >&2
-  exit 1
-fi
-
-if ! command -v go >/dev/null 2>&1; then
-  echo "Error: Go is not installed." >&2
-  exit 1
-fi
-
-cd "$GO_BACKEND_DIR"
-go mod download
-
-# Running inside the module installs the x/mobile revision pinned by go.mod.
-go install golang.org/x/mobile/cmd/gomobile
-export PATH="$(go env GOPATH)/bin:$PATH"
-gomobile init
-
-mkdir -p "$OUTPUT_DIR"
-gomobile bind \
-  -target=ios \
-  -tags ios \
-  -o "$OUTPUT_DIR/Gobackend.xcframework" \
-  .
-
-if [[ ! -d "$OUTPUT_DIR/Gobackend.xcframework" ]]; then
-  echo "Error: Gobackend.xcframework was not created." >&2
-  exit 1
-fi
-
-echo "Built $OUTPUT_DIR/Gobackend.xcframework"
+exec bash "$SCRIPT_DIR/build_rust_backend.sh" ios