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>

cmd/sift/extract_review_test.go

@@ -0,0 +1,11 @@
+package main
+
+import "testing"
+
+// P5c owner review: a client base URL with a path prefix keeps it, as fetch and search do.
+func TestExtractURLKeepsBasePath(t *testing.T) {
+	got := extractURL("https://sift.example:8443/prefix/", extractOpts{File: "a.pdf", Pages: "2"})
+	if want := "https://sift.example:8443/prefix/v1/extract?pages=2"; got != want {
+		t.Fatalf("extractURL = %q, want %q", got, want)
+	}
+}

cmd/sift/extractcmd.go

@@ -79,13 +79,11 @@ func extractURL(base string, o extractOpts) string {
 	if mode != "" {
 		v.Set("mode", mode)
 	}
-	u, err := url.Parse(base)
-	if err != nil {
-		return base + "/v1/extract?" + v.Encode()
+	u := strings.TrimRight(base, "/") + "/v1/extract"
+	if q := v.Encode(); q != "" {
+		u += "?" + q
 	}
-	u.Path = "/v1/extract"
-	u.RawQuery = v.Encode()
-	return u.String()
+	return u
 }
 
 func extractMain() {