feat(warehouse-sources): configure a default sync frequency for newly discovered schemas

Is your feature request related to a problem?

• A source can discover many schemas. Every schema that discovery adds after the first setup starts on a six-hour cadence, which nobody chose.
• There is no per-source default. sync_old_schemas_with_new_schemas creates each new row with get_or_create and no cadence, so the row falls back to the model default of six hours: products/warehouse_sources/backend/models/external_data_schema.py and the create call.
• The bulk action does not close the gap. bulk_update_schemas takes explicit schema ids, so it only changes the schemas that exist when the user runs it (presentation/views/external_data_source.py).
• Cost: users who want a daily cadence must re-audit the whole schema list after every discovery run. On a source with dozens of schemas this is recurring manual work, and a missed schema keeps syncing four times more often than intended.
• The deprecated ExternalDataSource.sync_frequency field is not read as a default for new schemas (models/external_data_source.py).

Describe the solution you'd like

• A per-source default sync frequency that new schemas inherit when discovery creates them.
• Settable in the source settings UI and over the API, with the current six-hour value as the fallback when unset.
• Applied at row creation in sync_old_schemas_with_new_schemas, so a schema is never enabled on a cadence the user did not pick.

Describe alternatives you've considered

Additional context

• Relevant path: products/warehouse_sources/.
• products/warehouse_sources/backend/temporal/data_imports/sources/COVERAGE_GAPS.md does not list this. That file tracks vendor endpoints and tables a source does not expose yet, not sync cadence defaults.
• No production sync failure is established. Priority P3.
• Internal traceability: ticket #72263.

Created with PostHog Desktop from this inbox report.

Start with products/warehouse_sources/backend/models/external_data_schema.py, especially sync_old_schemas_with_new_schemas and the model default, then inspect presentation/views/external_data_source.py and the source settings/API entry points. Add a per-source default with the six-hour fallback, expose it in settings and the API, and ensure newly discovered schemas inherit it at creation.