Add phoenix-todo-app (ported from todo-app example)

[nshkrdotcom]

Overview

Phoenix.Sync port of the existing todo-app example, demonstrating ElectricSQL integration with Phoenix backend.

Changes from Original

• Phoenix Backend replaces the original backend, using Phoenix.Sync for embedded ElectricSQL
• Same React Frontend with @electric-sql/react hooks (minimal changes)
• Dual deployment modes: Production (single server) + Development (CORS-enabled)
• Organized structure: phoenix/ and frontend/ directories with unified start.sh

Key Improvements

• No separate Electric service needed (embedded via Phoenix.Sync)
• Single-server production deployment eliminates CORS complexity
• Better developer experience with organized project structure
• Comprehensive documentation and setup scripts

Usage

./start.sh  # Choose production or development mode

Testing

• ✅ All Phoenix tests passing (39 tests)
• ✅ ElectricSQL real-time replication working
• ✅ React frontend with live data sync identical to original
• ✅ Both deployment modes tested and working

This provides the same todo app functionality with a cleaner Phoenix.Sync architecture.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.30%. Comparing base (ad1994e) to head (ce2d92c).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2803      +/-   ##
==========================================
- Coverage   78.36%   78.30%   -0.06%     
==========================================
  Files         153      153              
  Lines        7183     7183              
  Branches      272      272              
==========================================
- Hits         5629     5625       -4     
- Misses       1552     1556       +4     
  Partials        2        2              

Flag	Coverage Δ
elixir	77.21% <ø> (-0.04%)	⬇️
elixir-client	68.58% <ø> (ø)
packages/experimental	87.73% <ø> (-1.23%)	⬇️
packages/react-hooks	86.30% <ø> (ø)
packages/typescript-client	92.41% <ø> (ø)
packages/y-electric	55.12% <ø> (ø)
postgres-140000	77.87% <ø> (-0.06%)	⬇️
postgres-170000	78.00% <ø> (ø)
sync-service	78.26% <ø> (-0.04%)	⬇️
typescript	84.36% <ø> (-0.19%)	⬇️
unit-tests	78.30% <ø> (-0.06%)	⬇️

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