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.

tests/conftest.py

@@ -0,0 +1,8 @@
+import os
+import sys
+
+SRC = os.path.join(
+    os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "src"
+)
+if SRC not in sys.path:
+    sys.path.insert(0, SRC)