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/api.go

@@ -74,6 +74,10 @@ type Server struct {
 	reqN, blockN, scrapeN atomic.Uint64
 
 	pdfPoolOnce sync.Once
+
+	// uploads bounds how many /v1/extract bodies (up to PDFMaxUpload each) are read into
+	// memory at once, ahead of the PDF pool's own limits.
+	uploads chan struct{}
 }
 
 func New(version string, g *guard.Guard, f *fetch.Fetcher, keys *auth.Store, log *slog.Logger) *Server {
@@ -90,6 +94,7 @@ func New(version string, g *guard.Guard, f *fetch.Fetcher, keys *auth.Store, log
 		OutCap:  200_000,
 		global:  make(chan struct{}, 8),
 		per:     map[string]chan struct{}{},
+		uploads: make(chan struct{}, maxUploads),
 	}
 	s.Prom = metrics.New(s)
 	s.Mux.HandleFunc("GET /healthz", s.healthz)