feat: replace Playwright extractor with yt-dlp subprocess

- Add instagram-extractor.ts: yt-dlp subprocess backend for Instagram
  caption extraction. No in-process browser state, maintained against
  Instagram frontend churn, supports cookies.txt for auth-walled reels.
- Add feature flag EXTRACTOR_BACKEND (ytdlp|playwright) in QueueProcessor
  so the old Playwright path remains available as fallback.
- Add 9 unit tests and 2 live-network integration tests for the new extractor.
- Dockerfile: install yt-dlp via pip3 alongside existing Chromium deps.
- docker-compose: expose EXTRACTOR_BACKEND env var (default: ytdlp).

Also in this commit:
- LLM: configurable per-request timeout via LLM_REQUEST_TIMEOUT_MS (default 120s);
  set maxRetries=0 to surface errors immediately; llama-swap /running health probe.
- QueueProcessor: thread progress callback through parser phase.
- LlmHealthIndicator: surface llama-swap loaded-model name.
- Logging: improve error serialization in queue-processor tests.
- .env.example: document llama-swap endpoint and model options.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/lib/server/extraction.ts

@@ -26,7 +26,14 @@ type CaptionCandidate = {
 	brCount: number;
 };
 
-export type ProgressEventType = 'status' | 'method' | 'retry' | 'error' | 'thumbnail' | 'complete';
+export type ProgressEventType =
+	| 'status'
+	| 'method'
+	| 'retry'
+	| 'error'
+	| 'thumbnail'
+	| 'complete'
+	| 'model_loading';
 
 export interface ProgressEvent {
 	type: ProgressEventType;