feat: add bounty search, CSV import, templates, and schedules

[codeX-james]

Summary

• add a dedicated ranked and paginated bounty search endpoint
• add an admin-authenticated CSV bulk import with per-row validation results
• seed reusable bounty templates and apply defaults with request overrides
• add admin-managed recurring schedules and a background scheduler for automatic bounty creation
• document the new API surface and add focused integration coverage

Validation

• npm run typecheck
• ESLint on all changed TypeScript files
• 67 passing targeted backend tests covering the new features, pagination, auth, and OpenAPI contracts

Closes #783
Closes #784
Closes #785
Closes #786

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