feat(fonts): wire font extraction into capture pipeline + blob serving route

- capture.rs: add archive_id: Option<&str> to perform_capture; when Some,
  call font_extractor::extract_and_rewrite before hashing HTML, register
  each font as a deduplicated blob + 'font' artifact
- main.rs: pass None as archive_id (CLI keeps fonts embedded)
- routes.rs: add GET /api/archives/:id/blobs/:sha256 (serve_blob handler),
  pass Some(&archive_id) to perform_capture in capture_handler,
  add ApiError::internal constructor

crates/archivr-cli/src/main.rs

@@ -63,7 +63,7 @@ fn main() -> Result<()> {
                 }
             };
             let archive_paths = archive::read_archive_paths(&archive_path)?;
-            let result = archivr_core::capture::perform_capture(&archive_paths, path)?;
+            let result = archivr_core::capture::perform_capture(&archive_paths, path, None)?;
             println!("Archived: run {}", result.run_uid);
             Ok(())
         }