v0.40.0 — remove the metrics dashboard from the package (not a Verel feature)

Reverts the v0.39.0 decision to ship the metrics dashboard inside verel. The dashboard is a maintainer tool — it tracks the author's own projects' adoption and reads GitHub traffic data (which needs push access via local gh auth), so it has no use for anyone who installs Verel.

• Removed verel.dashboard and the verel-dashboard console script from the package.
• The dashboard lives under tools/metrics_dashboard.py again (not packaged); it keeps the fail-closed auth+TLS hardening and may be extracted into its own project later.
• No API a Verel user relied on is affected.

v0.39.0 is yanked on PyPI/TestPyPI — install verel>=0.40.0. (Package versions are immutable; yank, not delete, is the correct supersede.)

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