Add a QR code endpoint for PLM addresses

GET /qr/{address} renders the address as a PNG QR code (qrcode[pil]),
gated by a bech32-shaped regex since it's otherwise unauthenticated —
the address itself isn't sensitive, but this keeps it from being used
as an arbitrary text-to-QR service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

pyproject.toml

@@ -13,6 +13,7 @@ dependencies = [
     "pyjwt>=2.7",
     "cryptography>=43.0",
     "embit>=0.7",
+    "qrcode[pil]>=7.4",
 ]
 
 [project.optional-dependencies]