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/tests/transcribe_test.rs

@@ -58,7 +58,7 @@ async fn whisper_client_posts_multipart_and_returns_text() {
     Mock::given(method("POST"))
         .and(path("/asr"))
         .and(query_param("output", "txt"))
-        .and(query_param("encode", "false"))
+        .and(query_param("language", "de"))
         .respond_with(ResponseTemplate::new(200).set_body_string(expected))
         .expect(1)
         .mount(&server)