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.

.gitignore

@@ -37,11 +37,8 @@ AGENTS.md
 /bun.lock
 /package-lock.json
 
-# Go backend build outputs
-/go_backend/*.aar
-/go_backend/*.jar
-/go_backend/*.exe
-/go_backend/*.xcframework/
+# Rust backend build outputs and generated native bindings
+/rust_backend/target
 
 # Android build state and signing material
 /android/.gradle/
@@ -57,6 +54,7 @@ AGENTS.md
 /android/app/*.jks
 
 # iOS generated state
+/ios/build/
 /ios/Frameworks/
 /ios/Pods/
 /ios/.symlinks/
@@ -73,6 +71,8 @@ DerivedData/
 *.mobileprovision
 *.p12
 *.pem
+# Public ISRG roots embedded by the Rust network library.
+!/rust_backend/crates/network/src/roots.pem
 
 # Local research and design material
 /design/
@@ -81,10 +81,8 @@ DerivedData/
 /tool/
 /spotiflac_android/
 
-# Documentation published separately. Keep the in-repo extension contract
-# available because contribution templates link to it.
-/docs/*
-!/docs/EXTENSION_DEVELOPMENT.md
+# Local documentation, migration archives, fixtures, and benchmark evidence.
+/docs/
 
 # Temporary files and logs
 /.tmp/