api: NewBatchRunner — the batch fetch adapter, retry on busy, outcome metrics

Wire the batch runner to fetchTarget and feed the sift_batch_* metrics.
New internal/api/batch.go: NewBatchRunner overwrites cfg.Hooks (Job, Target,
JobSeconds, Depth, Inflight → s.Prom) and sets the runner's FetchFunc to
batchFetch, which tags the context as a batch fetch, runs under requestDeadline,
and maps fetchTarget's errors to what the runner retries (429/503 →
ErrRetryLater) and what the metrics classify (batchOutcome). Added the Batch
field to Server (nil → task 07 answers 503 "batch disabled") and, in
extractPDF, charged the PDF pool to c.Name+"/batch" under the batch mark so a
batch never takes a caller's interactive slot while the single-flight key is
unchanged. No HTTP endpoints.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

internal/api/pdf.go

@@ -58,7 +58,11 @@ func (s *Server) extractPDF(ctx context.Context, c *auth.Caller, res *fetch.Reso
 		return v, err
 	}
 	start := time.Now()
-	v, err := s.pool().Do(ctx, s.callerClass(c)+"|"+res.SHA256(), c.Name, fn)
+	caller := c.Name
+	if isBatchFetch(ctx) {
+		caller = c.Name + "/batch"
+	}
+	v, err := s.pool().Do(ctx, s.callerClass(c)+"|"+res.SHA256(), caller, fn)
 	total := time.Since(start)
 	// The job's own extraction time is what this call measured it to be; the
 	// rest of Do is time spent waiting for a slot.