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.go

@@ -27,6 +27,17 @@ func (s *Server) v1extract(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	select {
+	case s.uploads <- struct{}{}:
+		defer func() { <-s.uploads }()
+	case <-time.After(uploadWait):
+		w.Header().Set("Retry-After", "10")
+		writeErr(w, 429, "busy", "")
+		return
+	case <-r.Context().Done():
+		return
+	}
+
 	body, rerr := io.ReadAll(r.Body)
 	if rerr != nil {
 		var me *http.MaxBytesError
@@ -105,7 +116,7 @@ func (s *Server) v1extract(w http.ResponseWriter, r *http.Request) {
 		content = full
 	}
 
-	var warn []string
+	warn := []string{}
 	var digMeta map[string]any
 	var digestMS int64
 	if mode == "digest" {
@@ -192,6 +203,14 @@ func (s *Server) v1extract(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, 200, out)
 }
 
+// maxUploads is how many uploads may be in memory at once, and uploadWait how long a further
+// one waits for a slot before a 429: at 100 MiB each, a few are all a 1 GiB service can hold
+// beside the extraction children.
+const (
+	maxUploads = 2
+	uploadWait = 5 * time.Second
+)
+
 // pdfMaxUpload is the /v1/extract upload cap: the configured value, or the default when unset.
 func (s *Server) pdfMaxUpload() int64 {
 	if s.PDFMaxUpload > 0 {