refactor: collapse the single-implementation backend abstraction, fix device resolution

isr.py carried a plugin architecture — AudioBackend ABC, a _backend_classes
registry, _discover_backends, available_backends/get_backend, a
preferred_backend config knob — for exactly one implementation. It was
also the source of three real bugs:

* find_device() returned None before reaching the ALSA PCM-name fallback
  whenever `arecord -l` enumerated nothing, so the documented
  `device = shared_mic` (dsnoop from asound.conf) case failed outright on
  any host with no hardware card. The fallback is the whole point: those
  PCMs never appear in `arecord -l`.

* `device = monitor` could never resolve. ALSA has no generic monitor
  source and list_devices hardcoded is_monitor=False, so the keyword —
  advertised in config.example.ini as the "EASY" way to capture system
  audio — only ever produced "device not found". Removed; it is a
  PulseAudio idea that outlived the PulseAudio backend.

* list_all_devices() deduplicated by lowercased name "in case a device
  appears in multiple backends". With one backend what it actually did
  was delete the second of two identical USB cards. Dedup dropped.

find_device() now always returns a device, so _resolve_device's
unreachable "Available devices:" error is gone; an unknown spec is
passed to ALSA verbatim and logged. AudioDevice loses backend/
is_monitor plus channels/sample_rate — the latter were hardcoded 2/44100
and only ever printed by --list-devices, i.e. numbers with no bearing on
what gets recorded.

--list-devices now says outright that it lists hardware cards only, which
is where the shared_mic confusion started.

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

config.example.ini

@@ -67,16 +67,14 @@ format = auto
 #
 # # Device selection (use one of these methods):
 # #   device = default    - Use system default input device
-# #   device = monitor    - Auto-select first monitor/loopback source (for system audio)
 # #   device = <name>     - Match device by partial name (e.g., "Stereo Mix")
 # #   device = <id>       - Use exact device ID from --list-devices
+# #   device = <pcm>      - Any ALSA PCM name from asound.conf (e.g. shared_mic);
+# #                         these never show up in --list-devices but work anyway
 # #
 # # To list available devices, run: python isr.py --list-devices
 # device = default
 #
-# # Audio backend (only 'alsa' is supported):
-# # backend = alsa
-#
 # # Sample rate in Hz (common: 44100, 48000, 96000)
 # sample_rate = 44100
 #
@@ -122,17 +120,10 @@ format = auto
 #
 # Run: python isr.py --list-devices   (or: arecord -l) to list devices.
 #
-# EASY: Use the "monitor" keyword to capture system/loopback audio:
-# [system_audio]
-# type = soundcard
-# device = monitor
-# format = wav
-#
-# MANUAL: Specify a device by ALSA hardware ID from --list-devices:
+# Specify a device by ALSA hardware ID from --list-devices:
 # [usb_mic]
 # type = soundcard
 # device = hw:1,0
-# backend = alsa
 # sample_rate = 44100
 # channels = 2
 # format = flac
@@ -148,7 +139,6 @@ format = auto
 # [usb_mic_shared]
 # type = soundcard
 # device = shared_mic
-# backend = alsa
 # sample_rate = 48000
 # channels = 2
 # format = flac