Refactor user config validation and add transcript display

Introduces a dedicated function `validate_and_index_users` to
encapsulate the logic for validating user configurations, checking for
duplicate slugs and API keys. This function returns a `Result` to handle
errors gracefully, and its usage in `load_users` is updated to panic
with a more informative error message.

Additionally, this commit modifies the web routing to fetch and display
audio transcripts. When scanning recordings, it now attempts to read a
corresponding `.transcript.txt` file. If found, the transcript content
is included in the `RecordingView` and rendered in the `cases.html`
template. If the transcript file is not found, a "Transcription pending"
message is displayed.

The `transcribe` function in `whisper.rs` is updated to explicitly set
the language to German (`language=de`), which can improve transcription
accuracy by skipping language detection. The `encode=false` query
parameter has been removed, as the service is designed to handle encoded
audio.

Finally, tests have been added for the new `validate_and_index_users`
function to ensure duplicate slugs and API keys are correctly rejected.
Integration tests in `web_test.rs` have been updated to verify the
rendering of transcripts and the pending status.

server/src/transcribe/whisper.rs

@@ -27,8 +27,10 @@ impl std::error::Error for WhisperError {}
 
 /// POST the audio file to whisper-asr-webservice and return the transcript text.
 ///
-/// Uses `output=txt` for a plain-text response and `encode=false` to skip
-/// whisper's internal ffmpeg pass — we remux ourselves upstream.
+/// `output=txt` → plain-text response. `language=de` pins German so the model
+/// skips language detection. We send AAC-in-mp4 and let the service decode it
+/// internally (the default `encode=true`) — passing `encode=false` would tell
+/// the service "this is already raw PCM", which ours isn't.
 pub async fn transcribe(
     client: &reqwest::Client,
     whisper_url: &str,
@@ -48,7 +50,7 @@ pub async fn transcribe(
         .map_err(WhisperError::Http)?;
     let form = Form::new().part("audio_file", part);
 
-    let url = format!("{}/asr?output=txt&encode=false", whisper_url.trim_end_matches('/'));
+    let url = format!("{}/asr?output=txt&language=de", whisper_url.trim_end_matches('/'));
     debug!(%url, "posting to whisper");
 
     let response = client