RDM card reader: print card events, select the right port, serve taps to a web app

Card reading (rdm6300 + pyserial) printing card inserted/removed with the
full card data, plus a --serve mode that publishes taps over HTTP so a web
page can authenticate a tap.

The bridge reports card ids only - mapping a card to a user stays on the
server. The tap is latched for --tap-ttl seconds, including after the card is
lifted, and is single use, so a page reload cannot replay one tap into two
logins.

Port selection identifies the reader by serial number or by-id path instead of
/dev/ttyUSBn, and refuses to guess when a selector matches several adapters.

deploy/ has the systemd unit, a udev rule for a stable /dev/rdm-reader, and
install.sh. Covered by unit tests plus PTY end-to-end tests.

LICENSE

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+MIT License
+
+Copyright (c) 2026
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.