feat(obs): add modality-aware traces and image route preview

[typelicious]

What changed

• add modality-aware metrics and filtering for chat, image_generation, and image_editing
• add POST /api/route/image for dry-run preview of image-generation and image-editing routing
• expose modality breakdowns in stats, traces, recent requests, dashboard filters, and dashboard cards

Why

• starts the v0.6.x modality line with operator-visible debugging and observability
• makes image routing easier to verify without sending real upstream requests

How verified

• PYTHONPYCACHEPREFIX="$PWD/.pycache" python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q
• ./.venv-check-313/bin/ruff check .
• ./.venv-check-313/bin/ruff format --check .
• /usr/bin/git diff --check