feat: Add REST API for Functions CRUD operations

[sunnymodi21]

Summary

This PR implements a comprehensive REST API for Functions with full CRUD operations, following the same patterns as existing controllers (SinkConsumer, HttpEndpoint).

Changes

Backend Implementation

• ✅ FunctionController with index, show, create, update, delete actions
• ✅ FunctionJSON for response serialization
• ✅ Transforms module enhanced with from_external_function/1 for input parsing
• ✅ Consumers module enhanced with improved update_function/3
• ✅ Function schema fixed unique constraint name
• ✅ Router updated with /api/functions routes

API Endpoints

GET    /api/functions              - List all functions
GET    /api/functions/:id_or_name  - Get function by ID or name
POST   /api/functions              - Create new function
PUT    /api/functions/:id_or_name  - Update function by ID or name
DELETE /api/functions/:id_or_name  - Delete function by ID or name

Function Types Supported

• ✅ Filter - Filter which messages to process
• ✅ Transform - Modify message structure
• ✅ Enrichment - Enrich with additional data
• ✅ Path - Extract data from specific path
• ✅ Routing - Dynamically route messages

Features

• ✅ Flexible input format (supports both flat and nested structures)
• ✅ ID and name-based lookups
• ✅ Account-scoped operations for security
• ✅ Proper validation and error handling
• ✅ Follows existing controller patterns

Tests

• ✅ 19/19 tests passing for new FunctionController
• ✅ 1299/1299 total tests passing - no regressions
• ✅ Comprehensive coverage:
  • All CRUD operations
  • All 5 function types
  • ID and name-based lookups
  • Account isolation
  • Validation and error handling

Documentation

• ✅ 5 API reference docs (create, list, get, update, delete)
• ✅ Updated docs.json navigation
• ✅ Implementation guide (FUNCTIONS_REST_API.md)
• ✅ Test scripts for validation

Code Quality

• ✅ All code formatted with mix format
• ✅ No compiler warnings
• ✅ mix compile --warnings-as-errors passes
• ✅ Follows CONTRIBUTING.md guidelines
• ✅ Follows STYLE.md for documentation

Testing

All tests pass:

mix test test/sequin_web/controllers/function_controller_test.exs
# 19 tests, 0 failures

mix test --only unboxed
# 1299 tests, 0 failures

Test the API manually:

export API_TOKEN="your-token"
./test-function-api.sh

Example Usage

Create a filter function:

curl -X POST "http://localhost:7376/api/functions" \
  -H "Authorization: Bearer YOUR_API_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "my-filter",
    "description": "Filter records with value > 40",
    "type": "filter",
    "code": "def filter(action, record, changes, metadata) do\n  record[\"value\"] > 40\nend"
  }'

List all functions:

curl -X GET "http://localhost:7376/api/functions" \
  -H "Authorization: Bearer YOUR_API_TOKEN"

Files Changed

New Files (8):

• lib/sequin_web/controllers/function_controller.ex
• lib/sequin_web/controllers/function_json.ex
• test/sequin_web/controllers/function_controller_test.exs
• docs/management-api/functions/create.mdx
• docs/management-api/functions/list.mdx
• docs/management-api/functions/get.mdx
• docs/management-api/functions/update.mdx
• docs/management-api/functions/delete.mdx

Modified Files (5):

• lib/sequin/transforms/transforms.ex
• lib/sequin/consumers/consumers.ex
• lib/sequin/consumers/function.ex
• lib/sequin_web/router.ex
• docs/docs.json

Checklist

• Code follows project style guidelines
• All tests passing
• No compiler warnings
• Documentation created
• Follows existing patterns
• Ready for review

closes #2043