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.

server/tests/oneliners_api_test.rs

@@ -9,8 +9,8 @@ use filetime::FileTime;
 use tower::util::ServiceExt;
 
 use common::{
-    TestConfig, body_json, get_with_api_key, get_with_api_key_if_none_match, header_str,
-    seed_oneliner_ready, seed_oneliner_state, seed_recording_with_age, test_user,
+    CLOSE_MARKER, TestConfig, body_json, get_with_api_key, get_with_api_key_if_none_match,
+    header_str, seed_oneliner_ready, seed_oneliner_state, seed_recording_with_age, test_user,
     test_user_with_window_hours,
 };
 
@@ -56,7 +56,7 @@ fn seed(
 }
 
 fn mark_deleted(case_dir: &Path) {
-    std::fs::write(case_dir.join(".closed"), "{}").unwrap();
+    std::fs::write(case_dir.join(CLOSE_MARKER), "{}").unwrap();
 }
 
 #[tokio::test]