P5c review: bound concurrent uploads; warnings is a list; sift extract keeps a base-URL path

Owner review of p5c (tasks 01-02 by Ornith):
- extract.go: an upload (up to 100 MiB) is read whole before the PDF pool's limits apply, so
  any number of concurrent uploads could buffer at once in a 1 GiB service. At most 2 bodies
  are read at a time now; a third waits up to 5 s, then 429 busy with Retry-After 10.
- extract.go: `warnings` was null when empty; it is [] as in /v1/fetch.
- extractcmd.go: extractURL replaced the client base URL's path; it now appends /v1/extract
  like fetch and search do.
Regression tests in extract_review_test.go (api and cmd/sift).

End-to-end (local serve -> Dixie; sift extract with SIFT_CLIENT): Llama 2 paper, 13.6 MB /
77 pages: --pages 1 ok, repeat served from the content-hash cache, --grep 2 hits, --question
answered with page citations in 16.8 s. Log lines carry bytes/pages/mode/cached only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/extract_review_test.go

@@ -0,0 +1,42 @@
+package api
+
+// P5c owner review: regression tests for /v1/extract.
+
+import (
+	"net/http"
+	"testing"
+	"time"
+)
+
+// Uploads are buffered whole (up to 100 MiB) before the PDF pool sees them, so the number read
+// at once is bounded: a third concurrent upload waits, then is refused busy.
+func TestExtractUploadsBounded(t *testing.T) {
+	r := newPDFRig(t)
+	for i := 0; i < cap(r.s.uploads); i++ {
+		r.s.uploads <- struct{}{}
+	}
+	start := time.Now()
+	code, out := r.extract(t, r.fleet, "", threePDF(), "application/pdf")
+	if code != http.StatusTooManyRequests {
+		t.Fatalf("upload with every slot taken: %d %v, want 429", code, out)
+	}
+	if d := time.Since(start); d < uploadWait-100*time.Millisecond {
+		t.Fatalf("refused after %v, want a wait of about %v first", d, uploadWait)
+	}
+	<-r.s.uploads
+	if code, out := r.extract(t, r.fleet, "", threePDF(), "application/pdf"); code != 200 {
+		t.Fatalf("upload with a free slot: %d %v", code, out)
+	}
+}
+
+// The envelope's warnings are a list, never null, as in /v1/fetch.
+func TestExtractWarningsIsAList(t *testing.T) {
+	r := newPDFRig(t)
+	code, out := r.extract(t, r.fleet, "", threePDF(), "application/pdf")
+	if code != 200 {
+		t.Fatalf("%d %v", code, out)
+	}
+	if _, ok := out["warnings"].([]any); !ok {
+		t.Fatalf("warnings = %#v, want a list", out["warnings"])
+	}
+}