feat(httpserver,log): /healthz, graceful shutdown, slog constructor

Implements internal/httpserver and internal/log.

httpserver (forgejo-mcp-broker-8ei):
- Server struct owns the HTTP lifecycle; Run(ctx) blocks, Handler() returns
  the composed handler for unit tests
- GET /healthz returns JSON with status, version, git_revision, build_date,
  and store probe result. Returns 503 when the store reports unhealthy
- Signal handling delegated to the caller via ctx cancellation — main wires
  signal.NotifyContext, httpserver just responds to Done()
- Graceful shutdown with a configurable deadline (default 10s). When the
  deadline expires, falls back to http.Server.Close() so lingering
  connections are forcibly terminated — http.Server.Shutdown alone never
  interrupts active connections
- ExtraHandler extension point for the OAuth + MCP routes that land in
  phase 2 and phase 5, so the server doesn't need to be re-plumbed later

log:
- Small slog wrapper: New(w, debug) returns a JSON logger that stamps every
  record with service/version/git_rev for correlation across deployments
- Discard() helper for tests

Tests: 97.9% coverage on httpserver (all health states, wrong-method,
ExtraHandler dispatch, ctx-cancel shutdown, shutdown-deadline force-close
of hanging requests, missing-field errors), 100% on log.

Closes forgejo-mcp-broker-8ei.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/log/log.go

@@ -0,0 +1,32 @@
+// Package log constructs the process-wide structured logger.
+//
+// Every record carries the broker's build info so aggregated log backends can
+// correlate events across versions and deployments.
+package log
+
+import (
+	"io"
+	"log/slog"
+
+	"kode.naiv.no/olemd/forgejo-mcp-broker/internal/buildinfo"
+)
+
+// New returns a JSON slog.Logger writing to w. When debug is true the
+// logger starts at Debug level, otherwise at Info.
+func New(w io.Writer, debug bool) *slog.Logger {
+	level := slog.LevelInfo
+	if debug {
+		level = slog.LevelDebug
+	}
+	h := slog.NewJSONHandler(w, &slog.HandlerOptions{Level: level})
+	return slog.New(h).With(
+		slog.String("service", "fjmcp-broker"),
+		slog.String("version", buildinfo.Version),
+		slog.String("git_rev", buildinfo.GitRevision),
+	)
+}
+
+// Discard returns a logger that drops every record. Useful in tests.
+func Discard() *slog.Logger {
+	return slog.New(slog.DiscardHandler)
+}