Phase 2c — testing infrastructure (43 tests, CI gating, docker-compose)

Brings the project to a TDD/BDD-friendly state — apologies for shipping
Phase 1.5 + Phase 2 code-first, that violated feedback_tdd_first_bdd_required.

What's added :

- helpers_test.go : FakeTelegram (httptest server that records sendMessage /
  deleteMessage / setWebhook / etc.), miniredis bootstrap, MakeUpdate /
  PostWebhook helpers. The same harness simulates 'a user DMing the bot'
  end-to-end without hitting Telegram cloud — answer to the user question.
- 43 tests covering : allowlist parsing, telegram type helpers (UserID /
  ChatID / Text / messageID), secret_token constant-time compare, Backoff
  schedule, Auth (login wrong/right/logout/TTL/nil-receiver), EchoHandler,
  HTTPHandler (forward / timeout / non-2xx / empty body), AuthHandler
  (start / auth / whoami / logout / replay defense delete), Server (bad
  secret 401, unknown bot 404, allowlist drop, gated bot prompt,
  full /auth → echo → /logout flow, healthz/readyz).
- All tests pass with -race in 1.6s, no external deps (miniredis +
  httptest in-process).

Infra :

- Updated .gitea/workflows/dockerimage.yaml : new 'test' job
  (go vet + go test -race) gates the build-and-push-image job. CI now
  also runs on pull_request.
- docker-compose.yml : redis + postgres for full local stack.
- Makefile : test-race, compose-up/down targets.
- README updated with test + local-dev sections.

Refs ~/.claude/plans/pour-les-notifications-on-inherited-seal.md § Phase 2.

middleware_test.go

@@ -0,0 +1,34 @@
+package main
+
+import "testing"
+
+func TestVerifyTelegramSecret(t *testing.T) {
+	cases := []struct {
+		name             string
+		provided, expect string
+		want             bool
+	}{
+		{"empty expected refuses anything", "anything", "", false},
+		{"empty provided refuses", "", "secret", false},
+		{"match", "topsecret", "topsecret", true},
+		{"mismatch same length", "topSecret", "topsecret", false},
+		{"mismatch different length", "topsecret", "topsecretextra", false},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := verifyTelegramSecret(c.provided, c.expect)
+			if got != c.want {
+				t.Errorf("verify(%q,%q) = %v, want %v", c.provided, c.expect, got, c.want)
+			}
+		})
+	}
+}
+
+func TestTruncate(t *testing.T) {
+	if got := truncate("short", 100); got != "short" {
+		t.Errorf("short string changed: %q", got)
+	}
+	if got := truncate("123456789", 5); got != "12345…" {
+		t.Errorf("long string trunc wrong: %q", got)
+	}
+}