Adopt the modern-di integration kit

[lesnik512]

Summary

modern-di 2.28.0 shipped modern_di.integrations — framework-agnostic primitives that formalize what this package's _enter_request and @inject/FromDI/auto_inject already hand-roll. This swaps the hand-rolled internals for kit calls. No public-API change; zero test-file changes.

Fifth of 13 planned adapter conversions (after starlette 2.2.0, fastapi 2.10.0, litestar 2.13.0, aiohttp 2.2.0). The simplest conversion so far: Flask has exactly one connection type and is fully synchronous, so _enter_request derives scope/context via integrations.bind(flask_request_provider, connection) directly — no classify_connection dispatch (nothing to dispatch across) and no Container context-manager block is introduced (Flask's before_request/teardown_appcontext stay two separate hooks, unchanged in shape).

• _FromDI/_parse_inject_params deleted; FromDI = integrations.from_di; inject composes integrations.parse_markers/resolve_markers.
• The three hand-written "__modern_di_injected__" string-attribute touch points become integrations.is_injected/integrations.mark_injected.
• Dead imports (dataclasses, T_co) removed.
• architecture/dependency-injection.md promoted to describe the new internals.
• Bumps the modern-di floor to >=2.28,<3.

Test plan

• just test-ci — 100% line coverage, all 9 tests passing, zero test-file changes
• just lint-ci — ruff, ty, check-planning all clean
• Per-task spec + quality review (4 tasks, all approved)
• Whole-branch review (approved, no findings)