Refactor codebase foundations for clearer boundaries

[QuentinHourdeaux]

Summary

This PR refactors Stackdraft codebase foundations around clearer human-facing boundaries and stricter shared standards.

• documents the intended backend/frontend structure and codebase standards
• splits backend definitions, core services, infrastructure adapters, and shared mechanics into explicit folders
• centralizes backend tagged errors and extracts reusable HTTP, SQLite, time, and validation helpers
• replaces the State repository naming with a core store contract and SQLite store implementation
• moves frontend API, async, and form-error mechanics into frontend/src/lib while keeping entity API clients under frontend/src/api

Why

The main goal is maintainability: make the codebase easier for humans to scan, extend, and review as Stackdraft grows beyond the first State endpoints. The refactor keeps domain/resource behavior visible at call sites while moving repeated mechanics into small shared helpers.

Impact

The public State API behavior is preserved except that GET /api/states now treats scope as an explicitly required single query parameter at the HTTP boundary. Existing frontend callers already use the single-scope form.

Validation

• deno task ci