navidrome_bridge: add dropbox mode writing straight to the request spool

Reading radio_conjurer.liq shows request.playlist is not a Liquidsoap playlist
at all - it is a drop box. queue_processing() runs every 60s, reads every line,
pushes each into request.queue() as a URI, then deletes the file and recreates
it empty. That changes what writing to it means, so the mode is now named and
documented for what it is.

BRIDGE_MODE=dropbox (with "playlist" kept as an alias) appends the exact
translated path to that file. Because the lines are pushed as URIs it is an
exact hand-off - no keyword search - and it involves neither betoniarka nor the
bot, just the file and Liquidsoap. The liq also puts requests_queue first in the
fallback and does not apply the check_next replay guard to it, so a request
interrupts the rotation and plays even if the track ran recently; both are now
documented rather than left to be discovered.

Fixes found while wiring this up:
* the existence check was unconditional while the library mount was documented
  as optional, so a drop-box-only setup could never queue anything. It is now
  BRIDGE_VERIFY_FILE_EXISTS (auto|true|false), defaulting to checking only when
  the library is actually visible;
* a missing parent dir was silently created, which would swallow requests into
  the container's own filesystem when the radio's data dir was not mounted. It
  is now a hard error naming the likely cause.

Verified: alias resolves; append/drain/append cycle against a simulation of the
liq's read-remove-recreate; both misconfigurations raise instead of silently
succeeding; drop-box-only path works with the library absent; api mode
unchanged, still prepending the sentinel that wyszukaj() discards.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

navidrome_bridge/compose.yaml

@@ -15,12 +15,14 @@ services:
       # Which starred tracks were already sent to the radio.
       - ./data:/data
 
-      # ONLY for BRIDGE_MODE=playlist: the radio's data dir, so the exact path
-      # can be appended to request.playlist. Leave commented out for the
-      # default "api" mode, which needs no shared filesystem at all.
+      # ONLY for BRIDGE_MODE=dropbox: the radio's data dir, so the exact path
+      # can be appended to request.playlist - the file radio_conjurer.liq
+      # drains every 60s. Leave commented out for the default "api" mode,
+      # which needs no shared filesystem at all.
       # - /srv/betoniarka/data:/srv/betoniarka/data
 
-      # Optional, and only useful in playlist mode: mounting the library here
+      # Optional, and only useful in dropbox mode: mounting the library here
       # (read-only) lets the bridge verify a translated path really exists
-      # before queueing it, turning a silent radio no-op into a clear error.
+      # before queueing it (BRIDGE_VERIFY_FILE_EXISTS=auto), turning a silent
+      # radio no-op into a clear error.
       # - /srv/betoniarka/music:/srv/betoniarka/music:ro