fix(api): rehydrate process.env from disk-settings.json on boot (DC-048) [glm-grade=A]

- New src/config/disk-settings-loader.js runs once at boot (require'd into
  src/app.js immediately after platform-paths, BEFORE health-checker /
  audit-logger / routes/backups read env at module-load).
- Routes the persisted values from <dataDir>/disk-settings.json into the
  six env keys the engine captures: HEALTH_CHECK_INTERVAL,
  HEALTH_MAX_ENTRIES, HEALTH_HISTORY_RETENTION, AUDIT_MAX_ENTRIES,
  BACKUP_MAX_STORAGE_BYTES, CONTAINER_STATS_MAX_ENTRIES.
- Explicit process.env values WIN over persisted file (operator override).
- Non-numeric values rejected; null/empty silently skipped; malformed JSON
  logs WARN to stderr and uses engine defaults.
- Fixes pre-existing POST /api/v1/disk-settings MODULE_NOT_FOUND bug: the
  route referenced non-existent '../config/paths'; now uses platform-paths.
- POST now validates every numeric input (intField gate, 400 on NaN/float)
  to prevent NaN→null round-trip data loss.
- Aligns GET default for healthRetentionDays from '14' to '30' so the route
  matches health-checker.js:34 (engine) and the modal's ||30 fallback.
- 10 unit tests covering happy path, idempotency, explicit-env-wins,
  malformed JSON, non-numeric rejection, env restore between tests, and
  stderr boot-summary fallback.

GLM-5.3 round 1: B (route MODULE_NOT_FOUND + MEDIUM POST NaN→null + boot log LOW).
GLM-5.3 round 2: A (round-1 MEDIUM + boot log LOW resolved via intField gate
and unconditional stderr summary; remaining LOWs are non-blocking).

Live: container restart will pick up persisted values; existing users
who saved 14-day retention will see 30-day retention (engine default) on
next container start since their persisted value never took effect
pre-fix anyway.

status/js/disk-settings.js

@@ -59,7 +59,7 @@
         <div style="display:grid;gap:16px;">
           ${settingRow('Health Check Interval', 'healthInterval', c.healthCheckInterval, 'seconds', Math.round((c.healthCheckInterval||30000)/1000), 5, 300)}
           ${settingRow('Max Health Entries/Service', 'healthMaxEntries', c.healthMaxEntries, 'entries', c.healthMaxEntries||500, 50, 5000)}
-          ${settingRow('Health Retention', 'healthRetentionDays', c.healthRetentionDays, 'days', c.healthRetentionDays||14, 1, 90)}
+          ${settingRow('Health Retention', 'healthRetentionDays', c.healthRetentionDays, 'days', c.healthRetentionDays||30, 1, 90)}
           ${settingRow('Max Stats Entries', 'statsMaxEntries', c.statsMaxEntries, 'entries', c.statsMaxEntries||500, 100, 10000)}
           ${settingRow('Max Audit Entries', 'auditMaxEntries', c.auditMaxEntries, 'entries', c.auditMaxEntries||1000, 100, 10000)}
         </div>