feat(settings): persist storage provider settings to DB, add export endpoint, enrich setup wizard

- dropbox/google_drive/onedrive save-settings: switch to DB as primary,
  .env write as best-effort (no longer fails when .env is absent)
- onedrive/google_drive update-settings: persist changes to DB alongside
  in-memory update; call notify_settings_updated()
- onedrive test_onedrive_token: persist rotated refresh token to DB
- settings_service: add get_settings_for_export() (db / effective modes)
- settings API: add GET /api/settings/export-env (admin-only, downloads .env)
- wizard: enrich settings with current values (DB > ENV > default) and
  value_source badges; pass setup_skipped to template; call
  notify_settings_updated() on save; add /setup/undo-skip route
- setup_wizard.html: pre-populate inputs with current_value; show
  DB/ENV/DEFAULT source badges; skip/undo-skip messaging
- settings.html: replace single Audit Log button with Setup Wizard link,
  Export .env dropdown, and Audit Log button group

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/utils/settings_service.py

@@ -1287,3 +1287,38 @@ def rollback_setting(
         )
         db.rollback()
         return False
+
+
+def get_settings_for_export(db: Session, source: str = "db") -> Dict[str, str]:
+    """
+    Collect settings for export as environment variables.
+
+    Args:
+        db: Database session
+        source: ``"db"`` to export only database-persisted settings (default);
+                ``"effective"`` to export the full current runtime configuration
+                (DB overrides ENV overrides application defaults) for every key
+                listed in SETTING_METADATA.
+
+    Returns:
+        Ordered dict mapping uppercase ENV variable names to their string values.
+        Sensitive values are included (the caller is responsible for access control).
+    """
+    if source == "effective":
+        from app.config import settings as app_settings
+
+        db_settings = get_all_settings_from_db(db)
+        result = {}
+        for key in sorted(SETTING_METADATA.keys()):
+            # DB wins, then live settings object (ENV/default)
+            if key in db_settings and db_settings[key] is not None:
+                value = db_settings[key]
+            else:
+                value = getattr(app_settings, key, None)
+            if value is not None:
+                result[key.upper()] = str(value)
+        return result
+    else:
+        # DB only
+        db_settings = get_all_settings_from_db(db)
+        return {k.upper(): v for k, v in sorted(db_settings.items()) if v is not None}