tlsprobe: add VerifyWithExponentialBackoff + rewire all connectors' runPostDeployVerify

Closes Top-10 fix #8 of the 2026-05-02 deployment-target audit
re-run (see cowork/deployment-target-audit-2026-05-02-rerun/
RESULTS.md). Pre-fix, every connector's runPostDeployVerify used
linear backoff (default 3 attempts × 2s linear waits). Linear
backoff misbehaves under load-balanced rollouts: the verify
probe hits a random LB-backed pod, and 3 × 2s often falls into
the worst case where match-fingerprint pods stop responding by
attempt 3 due to LB session-stickiness cycles.

This commit:

1. New shared helper internal/tlsprobe/retry.go::
   VerifyWithExponentialBackoff. Default 3 attempts; 1s initial,
   16s cap. Doubling pattern: 1s → 2s → 4s → 8s → 16s. probe
   func(ctx) error signature so connectors compose
   handshake + fingerprint-compare into one lambda.

2. Each connector's runPostDeployVerify (nginx, apache, haproxy,
   traefik, envoy, postfix, dovecot) rewired to call the
   shared helper. Per-connector signature unchanged.

3. New PostDeployVerifyMaxBackoff time.Duration field added to
   each connector's Config. Operators preserving V2 linear
   behavior set PostDeployVerifyMaxBackoff equal to
   PostDeployVerifyBackoff.

4. Tests:
   - tlsprobe/retry_test.go: TestVerifyWithExponentialBackoff_
     GrowthAndCap + TestVerifyWithExponentialBackoff_
     StopsOnFirstSuccess + TestVerifyWithExponentialBackoff_
     CtxCancellation.
   - One Test<Connector>_VerifyExponentialBackoff_
     GrowsBetweenAttempts per connector (6 total across
     postfix, nginx, apache, haproxy; traefik and envoy
     connectors use unique test signatures so test wiring
     deferred to future unification).

5. docs/deployment-atomicity.md Section 4 updated:
   'linear backoff' → 'exponential backoff (1s → 16s cap)';
   YAML example shows the new field.

Backward-compat note: PostDeployVerifyBackoff was interpreted as
the linear interval pre-fix; post-fix it's interpreted as the
initial backoff (which doubles each attempt). Operators using
the default value (2s) see waits of 2s → 4s → 8s instead of
2s → 2s → 2s. For LB-rollout cases this is the intended
behavior; for single-target deploys the wall-clock is slightly
longer (12s vs 6s for 3 attempts). Operators preserving V2
linear semantics: set PostDeployVerifyMaxBackoff equal to
PostDeployVerifyBackoff.

Verified locally:
- gofmt clean.
- go test -short -count=1 ./internal/tlsprobe/...
  ./internal/connector/target/{postfix,nginx,apache,haproxy}/... green.

Audit reference: cowork/deployment-target-audit-2026-05-02-rerun/
RESULTS.md Top-10 fix #8.

internal/tlsprobe/retry.go

@@ -0,0 +1,68 @@
+// Copyright (c) 2025 Certctl Contributors <certctl@proton.me>
+//
+// SPDX-License-Identifier: BSL-1.1
+// See COPYING for license details.
+
+package tlsprobe
+
+import (
+	"context"
+	"time"
+)
+
+// RetryConfig holds parameters for exponential-backoff retries.
+// Zero values use defaults: 3 attempts, 1s initial, 16s max.
+type RetryConfig struct {
+	Attempts       int           // total attempts; 0 = use 3 default
+	InitialBackoff time.Duration // base; 0 = use 1 * time.Second default
+	MaxBackoff     time.Duration // cap; 0 = use 16 * time.Second default
+}
+
+// VerifyWithExponentialBackoff calls the probe at most cfg.Attempts times,
+// waiting cfg.InitialBackoff, 2*InitialBackoff, 4*InitialBackoff, ... capped at
+// cfg.MaxBackoff between consecutive attempts. Returns nil on first probe success;
+// returns the last attempt's error on full exhaustion.
+//
+// The probe function returns:
+//   - nil error on success → return immediately, no further attempts.
+//   - non-nil error → wait the exponentially-growing backoff and retry.
+//
+// The ctx is checked between attempts; ctx cancellation aborts immediately.
+//
+// Top-10 fix #8 of the 2026-05-02 deployment-target audit re-run.
+func VerifyWithExponentialBackoff(ctx context.Context, cfg RetryConfig, probe func(ctx context.Context) error) error {
+	attempts := cfg.Attempts
+	if attempts <= 0 {
+		attempts = 3
+	}
+	initial := cfg.InitialBackoff
+	if initial <= 0 {
+		initial = 1 * time.Second
+	}
+	max := cfg.MaxBackoff
+	if max <= 0 {
+		max = 16 * time.Second
+	}
+
+	backoff := initial
+	var lastErr error
+	for i := 0; i < attempts; i++ {
+		if i > 0 {
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case <-time.After(backoff):
+			}
+			backoff *= 2
+			if backoff > max {
+				backoff = max
+			}
+		}
+		if err := probe(ctx); err == nil {
+			return nil
+		} else {
+			lastErr = err
+		}
+	}
+	return lastErr
+}