Extract dashboard API routes from app.py

[raychrisgdp]

Goal

Move dashboard data API routes out of app.py into a focused route module or Flask blueprint.

Candidate Scope

• /api/overview
• /api/models
• /api/daily
• /api/usage-history
• /api/refresh if it remains dashboard-data related

Constraints

• Preserve current response shapes and token/source wording.
• Keep simulated mode working.
• Keep route registration simple from app.py.
• Avoid changing aggregation rules in the same PR.

Verification

• uv run python -m unittest tests.test_app tests.test_readme -v
• uv run python -m py_compile app.py scripts/snapshot_dashboard.py
• scripts/review.sh