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/silent_case_empty_test.rs

@@ -26,7 +26,7 @@ use tempfile::tempdir;
 use wiremock::matchers::{method, path as wm_path};
 use wiremock::{Mock, MockServer, ResponseTemplate};
 
-use common::TestConfig;
+use common::{ONELINER_FILENAME, TestConfig};
 
 /// Seed an m4a plus a silent (0-byte) transcript sidecar — the on-disk
 /// shape of a recording that whisper classified as silence.
@@ -90,7 +90,7 @@ async fn silent_only_case_settles_to_empty_without_llm_call() {
 
     // Oneliner is persisted as `Empty`. Without the fix, the file would
     // either not exist or be mid-generation.
-    let path = case_dir.join("oneliner.json");
+    let path = case_dir.join(ONELINER_FILENAME);
     assert!(
         path.exists(),
         "expected oneliner.json to be written for silent-only case"