ci: add secret scanning for GitHub, Gitea, and local use

- .gitleaks.toml — extends gitleaks defaults with Pi-hole-specific rules
  for pwhash/totp_secret/app_pwhash; allowlists known-safe patterns
- .github/workflows/secret-scan.yml — GitHub Actions (full history scan)
- .gitea/workflows/secret-scan.yml — Gitea Actions (identical workflow)
- scripts/check-secrets.sh — shared runner used by both CI and local;
  supports --staged-only for pre-commit hook use; falls back to Docker
  if gitleaks isn't on PATH
- scripts/install-hooks.sh — installs pre-commit hook pointing at above
- pihole/sanitize-config.sh — redacts pwhash/totp_secret/app_pwhash
  in pihole.toml in-place before the file is committed
- pihole/pull-config.sh — updated to call sanitize-config.sh
  automatically after every pull so the repo stays clean by default

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XRzqNDrbnYR22ZgZj1Bg3s

.gitleaks.toml

@@ -0,0 +1,41 @@
+# Gitleaks configuration for debian-configuration repo.
+# Extends the default ruleset with Pi-hole-specific secret patterns.
+# https://github.com/gitleaks/gitleaks
+
+title = "debian-configuration secret scan"
+
+[extend]
+  useDefault = true
+
+# ── Custom rules ───────────────────────────────────────────────────────────────
+
+[[rules]]
+  id          = "pihole-pwhash"
+  description = "Pi-hole password hash (pihole.toml webserver.api.pwhash)"
+  regex       = '''pwhash\s*=\s*"[^"]{10,}"'''
+  tags        = ["pihole", "password"]
+
+[[rules]]
+  id          = "pihole-totp-secret"
+  description = "Pi-hole 2FA TOTP secret"
+  regex       = '''totp_secret\s*=\s*"[^"]{10,}"'''
+  tags        = ["pihole", "2fa"]
+
+[[rules]]
+  id          = "pihole-app-pwhash"
+  description = "Pi-hole app password hash"
+  regex       = '''app_pwhash\s*=\s*"[^"]{10,}"'''
+  tags        = ["pihole", "password"]
+
+# ── Allowlist ──────────────────────────────────────────────────────────────────
+
+[allowlist]
+  description = "Known-safe patterns in this repo"
+  regexes     = [
+    # TLS cert path reference — not the key itself
+    '''cert\s*=\s*"/etc/pihole/tls\.pem"''',
+  ]
+  paths       = [
+    # Example/template files are intentionally non-live
+    '''\.example$''',
+  ]