feat(templates): implement stream template system (#596)

[fytgian]

Summary

Closes Vera3289#596

Builds a complete stream template system: employers can save stream configurations as named templates, share them with their team, and instantiate streams directly from a template.

Changes

New files

• api/services/templateService.js — In-memory store with full CRUD, createFromStream (strips runtime fields employer/employee, keeps reusable params), owner-scoped listing that includes shared + default templates, share toggle, and 2 built-in default templates seeded at module load
• api/routes/streamTemplates.js — 8 REST endpoints with express-validator and swagger JSDoc

Modified files

• api/server.js — Mounts /v1/api/stream-templates behind authMiddleware

API Endpoints

Method	Path	Description
POST	/v1/api/stream-templates	Create template from scratch
POST	/v1/api/stream-templates/from-stream	Save existing stream params as template
GET	/v1/api/stream-templates	List own + shared + default templates
GET	/v1/api/stream-templates/:id	Get single template
PATCH	/v1/api/stream-templates/:id	Modify name, params, or shared flag
DELETE	/v1/api/stream-templates/:id	Delete template (own non-default only)
POST	/v1/api/stream-templates/:id/share	Toggle team sharing
POST	/v1/api/stream-templates/:id/create-stream	Instantiate stream from template

Acceptance Criteria

Criterion	Status
Template creation from stream	✅ POST /from-stream strips employer/employee, saves reusable params
Template listing	✅ GET / returns own + shared + defaults
Create stream from template	✅ POST /:id/create-stream with optional overrides object
Template modification	✅ PATCH /:id — name, params, shared
Template deletion	✅ DELETE /:id — owner-only, protects default templates
Sharing templates with team	✅ POST /:id/share + shared templates visible to all users in listing
Default templates library	✅ 2 defaults seeded: Monthly Salary USDC, Weekly Contractor USDC