refactor: consolidate bulk actions, URL assembly, and case-artefact filenames

Pulls three pattern groups into shared helpers so a rename or wire-format
tweak edits one file instead of 10+:

- BulkAction enum in doctate-common replaces the "close"/"analyze"/"reset"
  string literals that were duplicated between the bulk handler and 3
  attack/CSRF test files. FromStr preserves the exact "Unbekannte Aktion"
  error shape; the handler match is now exhaustive over the enum.
- join_url helper in doctate-common absorbs 7 identical
  trim_end_matches+format! call sites across client-core, client-desktop,
  server/transcribe (ollama, whisper), and server/analyze (llm).
- server/tests/common/artefacts.rs re-exports ONELINER_FILENAME
  (doctate-common), DOCUMENT_FILE + ANALYSIS_INPUT_FILE
  (doctate-server::analyze), and CLOSE_MARKER (doctate-server::paths) so
  test files reference the canonical name instead of inlining literals.

Also lifts client-desktop local duplication:
- paths.rs: project_path helper collapses 4 identical ProjectDirs chains
- main.rs: or_die helper replaces 4 eprintln!+exit(1) blocks
- app.rs: named RecordingContext struct replaces (Uuid, String) tuple
  at 3 sites around the ffmpeg-flush finalization path

Verification: 397 tests pass (baseline was 389; +8 for new unit tests on
BulkAction and join_url), 0 failed, 4 ignored (unchanged). clippy clean.

doctate-client-core/src/server_sync.rs

@@ -27,6 +27,7 @@ use doctate_common::constants::{
     API_KEY_HEADER, CONTENT_TYPE_AUDIO_MP4, FIELD_AUDIO, FIELD_CASE_ID, FIELD_RECORDED_AT,
     UPLOAD_PATH,
 };
+use doctate_common::join_url;
 use doctate_common::oneliners::{ONELINERS_PATH, OnelinersResponse};
 use reqwest::StatusCode;
 use tokio::sync::{mpsc, watch};
@@ -369,7 +370,7 @@ pub(crate) async fn post_upload(
         .text(FIELD_RECORDED_AT, upload.recorded_at.clone())
         .part(FIELD_AUDIO, audio_part);
 
-    let url = format!("{}{}", config.server_url.trim_end_matches('/'), UPLOAD_PATH);
+    let url = join_url(&config.server_url, UPLOAD_PATH);
 
     let resp = match http
         .post(&url)
@@ -406,11 +407,7 @@ pub(crate) async fn poll_once(
     cache: &SnapshotCache,
     etag: &mut Option<String>,
 ) -> Result<PollOutcome, PollError> {
-    let url = format!(
-        "{}{}",
-        config.server_url.trim_end_matches('/'),
-        ONELINERS_PATH,
-    );
+    let url = join_url(&config.server_url, ONELINERS_PATH);
 
     let mut req = http.get(&url).header(API_KEY_HEADER, &config.api_key);
     if let Some(tag) = etag.as_deref() {