Dynamic README badge image

No auth — this URL is embedded in public READMEs. Renders a shields.io-style SVG badge with the tenant’s live gated-write count over the trailing 30 days (e.g. agent writes gated · 1.2k/30d), green when the tenant gated at least one write in the window, grey at zero. Only aggregate, tenant-opted-in data leaves the gate: no tenant IDs, names, keys, or receipt contents. Unknown or revoked tokens 404. Cached for one hour (Cache-Control: public, max-age=3600); badge views are rate-limited to 120/minute per token.