fix(ci): retry Vikunja lookups and surface self-approval merge failures

Implements: DEVX-164 REQ-1..3

- get_vikunja_task_title retries list_project_tasks up to 4 attempts
  with backoff on APIError/RequestException — a Vikunja restart window
  (404/502) no longer strands an otherwise-valid PR
- merge HTTP 405 now fetches PR reviews and, when no APPROVED review
  exists, explains the same-user self-approval rejection and the
  remediation (approve via a non-author account)
- regression tests for retry-then-success, retry exhaustion, missing
  task, and all three 405 review states

docs/specs/DEVX-164-docker-push-retry-historical.md

@@ -0,0 +1,34 @@
+# DEVX-164: Increase HTTP 500 retry count and backoff for docker push
+
+## Problem
+The HTTP 500 retry logic (DEVX-162, DEVX-163) works correctly — 3 retry
+attempts are made. But all 3 attempts fail because the Gitea registry's
+"offset mismatch" race condition needs more than ~15s to recover. The
+current backoff is 5s-20s with 3 attempts (total ~15s of waiting).
+
+## Approach
+Increase retry count from 3 to 5 and backoff from 5-20s to 10-60s,
+giving the registry up to ~2 minutes to recover. Add visible logging
+between retry attempts so the CI logs show the retry happening.
+
+REQ-1: Increase retry count from 3 to 5
+REQ-2: Increase backoff from 5-20s to 10-60s exponential
+REQ-3: Add visible logging between retry attempts (click.echo)
+REQ-4: All tests pass with 100% coverage
+
+## Test Plan
+- Unit tests verify retry count and backoff parameters
+- Unit tests verify logging output on retry
+- Manual: trigger build-images workflow and verify retries visible in logs
+
+## Deploy Plan
+- Merge to master
+
+## Rollback Plan
+- Revert the merge commit
+
+## Acceptance Criteria
+- [x] REQ-1: Increase retry count from 3 to 5
+- [x] REQ-2: Increase backoff from 5-20s to 10-60s exponential
+- [x] REQ-3: Add visible logging between retry attempts (click.echo)
+- [x] REQ-4: All tests pass with 100% coverage