fix(sync-mirror): tighten gh-pr-* regex + document SQL-integer-safety

Ganymede review nit on commit 1eb259d:

- Regex changed from [0-9]* (zero-or-more) to [0-9][0-9]* (one-or-more,
  portable BRE form of [0-9]+ that works on both GNU and BSD sed).
- Empty/non-numeric branches now fail at parse, not just at the empty-guard
  below — SQL-integer-safety load-bearing on the regex alone.
- Comment above the UPDATE notes the integer-validation invariants
  (INTEGER `number` column + regex-validated gh_pr_num) since bash sqlite3
  has no parametric binding.

Smoke tested: gh-pr-/foo, gh-pr-abc/foo no longer parse to non-empty.
gh-pr-90/main, gh-pr-4066/contrib/x, gh-pr-1/x all parse correctly.

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

deploy/sync-mirror.sh

@@ -367,6 +367,34 @@ print(json.dumps({'chat_id': sys.argv[4], 'text': msg, 'parse_mode': 'HTML'}))
 REPO_TAG="main"
 log "Starting sync cycle"
 
+# Step 0: self-heal any gh-pr-* PR rows missing github_pr.
+# Runs FIRST — before per-repo work (branch-mirror loop, auto-create-PR block).
+# Recovers from races/transient failures in Step 4.5's one-shot link UPDATE.
+# Idempotent: SELECT empty when clean, zero-cost path. Same SELECT/UPDATE
+# heals historical orphans (PR 4066 picked up on first cron tick post-deploy)
+# and future races on subsequent ticks. The branch name encodes the GitHub PR
+# number deterministically (gh-pr-{N}/...) so no API call is required.
+if [ -f "$PIPELINE_DB" ]; then
+    sqlite3 -separator '|' "$PIPELINE_DB" \
+        "SELECT number, branch FROM prs WHERE branch LIKE 'gh-pr-%' AND github_pr IS NULL;" \
+        2>/dev/null | while IFS='|' read -r pr_num branch; do
+        # Regex requires >=1 digit — empty/non-numeric branches fail to parse here,
+        # not just at the empty-guard below. Keeps SQL-integer-safety load-bearing
+        # on the regex alone. [0-9][0-9]* is the portable BRE form of [0-9]+,
+        # works on both GNU sed (VPS) and BSD sed (dev macs).
+        gh_pr_num=$(echo "$branch" | sed -n 's|^gh-pr-\([0-9][0-9]*\)/.*|\1|p')
+        [ -z "$gh_pr_num" ] && continue
+        # Both interpolated values are integer-validated upstream (pr_num from
+        # INTEGER `number` column, gh_pr_num from regex above). No parametric
+        # binding available in bash sqlite3 — safety relies on those invariants.
+        if sqlite3 "$PIPELINE_DB" \
+            "UPDATE prs SET github_pr = $gh_pr_num, source_channel = 'github' WHERE number = $pr_num;" \
+            2>/dev/null; then
+            log "self-heal: linked Forgejo PR #$pr_num -> GitHub PR #$gh_pr_num"
+        fi
+    done
+fi
+
 for entry in "${MIRROR_REPOS[@]}"; do
     # Read the 4 fields. `read` splits on $IFS (whitespace) by default.
     read -r forgejo_repo github_repo bare_path mode <<< "$entry"