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>

tests/fixtures/go_schema_v1.sql

@@ -0,0 +1,38 @@
+-- SQLite schema created by the original Go server (v1.0.0, internal/database/database.go).
+-- Rust must keep opening databases with exactly this schema; do not edit.
+CREATE TABLE IF NOT EXISTS users (
+  id TEXT PRIMARY KEY,
+  email TEXT NOT NULL UNIQUE COLLATE NOCASE,
+  password_hash TEXT NOT NULL,
+  created_at TEXT NOT NULL
+);
+CREATE TABLE IF NOT EXISTS refresh_tokens (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  token_hash TEXT NOT NULL UNIQUE,
+  user_id TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  expires_at TEXT NOT NULL,
+  created_at TEXT NOT NULL,
+  revoked_at TEXT
+);
+CREATE INDEX IF NOT EXISTS refresh_tokens_user_idx ON refresh_tokens(user_id);
+CREATE TABLE IF NOT EXISTS records (
+  user_id TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  entity TEXT NOT NULL,
+  record_id TEXT NOT NULL,
+  payload TEXT,
+  updated_at TEXT NOT NULL,
+  deleted INTEGER NOT NULL DEFAULT 0,
+  version INTEGER NOT NULL,
+  PRIMARY KEY (user_id, entity, record_id)
+);
+CREATE TABLE IF NOT EXISTS changes (
+  sequence INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  entity TEXT NOT NULL,
+  record_id TEXT NOT NULL,
+  payload TEXT,
+  updated_at TEXT NOT NULL,
+  deleted INTEGER NOT NULL,
+  version INTEGER NOT NULL
+);
+CREATE INDEX IF NOT EXISTS changes_user_sequence_idx ON changes(user_id, sequence);