Refactor recording metadata to JSON sidecar

Replaces the `.transcript.txt` sidecar with a structured `.json` file
for recording metadata. This change consolidates transcript text,
duration, and other potential metadata into a single, extensible JSON
object.

This also refactors the `TranscriptState` enum to better represent the
on-disk state (absence of file means pending) and the in-memory
representation. The `Transcript` enum now specifically models the
terminal outcomes of the transcriber (`Silent` or `Content`).

The commit includes updates to documentation, data structures, path
handling, and various tests to align with the new metadata format.

server/tests/common/mod.rs

@@ -27,13 +27,15 @@ pub mod users;
 // Re-exports for ergonomic `use common::*;` in most test files.
 pub use artefacts::{ANALYSIS_INPUT_FILE, CLOSE_MARKER, DOCUMENT_FILE, ONELINER_FILENAME};
 pub use config::{TestConfig, unique_tmpdir};
+pub use doctate_common::Transcript;
 pub use http::{
     body_bytes, body_json, body_string, csrf_form_post, form_post, get_with_api_key,
     get_with_api_key_if_none_match, get_with_cookie, header_opt, header_str, multipart_upload_body,
 };
 pub use seed::{
     past_rfc3339, seed_case, seed_oneliner_ready, seed_oneliner_state, seed_recording,
-    seed_recording_with_age, seed_recording_with_sidecars, write_closed_marker,
+    seed_recording_meta, seed_recording_with_age, seed_recording_with_sidecars,
+    write_closed_marker,
 };
 pub use session::{extract_session_cookie, login, login_request, login_with_csrf};
 pub use users::{