Remove the Go reference server; keep only Rust

The Go implementation is preserved in the v1.0.0 tag. Its observable
behaviour stays the API reference: responses recorded from the Go server
live in tools/compatibility_expected.txt, and tools/compatibility.py now
compares the Rust server against them (--update re-records after an
intentional change). The Go SQLite schema is frozen in
tests/fixtures/go_schema_v1.sql for the existing-database test.

Move the web client from internal/httpapi/web to web/, drop the Go CI job
and Go version pin, and update README, AGENTS.md and docs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/httpapi/web/manifest.webmanifest

@@ -1,13 +0,0 @@
-{
-  "name": "Balance — личные финансы",
-  "short_name": "Balance",
-  "description": "Self-hosted приложение для управления личными финансами",
-  "start_url": "/",
-  "scope": "/",
-  "display": "standalone",
-  "background_color": "#f4f5f8",
-  "theme_color": "#5b5bd6",
-  "icons": [
-    { "src": "/icon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any maskable" }
-  ]
-}