Phase 0: bootstrap the Warp homeserver

Scaffolds the server module and everything Phase 0 of PLAN.md calls for:
a binary that boots, serves its discovery endpoints and nothing else yet.

- config: one YAML file plus per-field WARP_* environment overrides,
  unknown fields rejected, validation that reports every problem at once
- logging and metrics: slog, a Prometheus registry, per-route HTTP
  instrumentation labelled by pattern rather than path
- httputil: the Matrix error envelope, error-returning handlers, CORS,
  request logging, panic recovery, and a router over http.ServeMux
- endpoints: /_matrix/client/versions, /.well-known/matrix/client,
  /.well-known/matrix/server (gated), /healthz, /metrics, and
  M_UNRECOGNIZED for everything else
- storage: driver registry, shared database/sql base, checksummed
  migration runner, sqlite and postgres drivers, pocketbase stub
- settings: server_settings table behind a cached service, so the
  federation and registration switches change without a restart
- federation: the gate that makes federation a runtime mode
- cmd/warp: serve, migrate, generate-keys, version
- Makefile, golangci-lint v2, GitHub Actions CI (Linux and Windows tests,
  race on Linux, storage tests against PostgreSQL, lint, boot smoke test),
  Dockerfile, example config and docs

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/internal/logging/logging.go

@@ -0,0 +1,52 @@
+// Package logging configures the process-wide slog logger.
+package logging
+
+import (
+	"fmt"
+	"io"
+	"log/slog"
+	"strings"
+)
+
+// Options mirrors the logging section of the config file.
+type Options struct {
+	Level     string
+	Format    string
+	AddSource bool
+}
+
+// ParseLevel maps a config string onto a slog level.
+func ParseLevel(s string) (slog.Level, error) {
+	switch strings.ToLower(strings.TrimSpace(s)) {
+	case "debug":
+		return slog.LevelDebug, nil
+	case "", "info":
+		return slog.LevelInfo, nil
+	case "warn", "warning":
+		return slog.LevelWarn, nil
+	case "error":
+		return slog.LevelError, nil
+	default:
+		return 0, fmt.Errorf("unknown log level %q", s)
+	}
+}
+
+// New builds a logger writing to w. It does not install it as the default.
+func New(w io.Writer, opts Options) (*slog.Logger, error) {
+	level, err := ParseLevel(opts.Level)
+	if err != nil {
+		return nil, err
+	}
+	handlerOpts := &slog.HandlerOptions{Level: level, AddSource: opts.AddSource}
+
+	var h slog.Handler
+	switch strings.ToLower(strings.TrimSpace(opts.Format)) {
+	case "", "text":
+		h = slog.NewTextHandler(w, handlerOpts)
+	case "json":
+		h = slog.NewJSONHandler(w, handlerOpts)
+	default:
+		return nil, fmt.Errorf("unknown log format %q", opts.Format)
+	}
+	return slog.New(h), nil
+}