Initial scaffold: FocusFlow ADHD Task Manager backend

Dart Shelf API with modules: auth (JWT + PBKDF2), tasks (CRUD + dopamine scorer),
streaks (forgiveness + freeze), rewards (variable reward engine), time perception,
sync (offline-first push/pull), rooms (body doubling placeholder).
Includes DB migration (001_initial_schema.sql) and Docker Compose.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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