fix(downloader): report aria2 per-peer progress; perf(ci): scope server COPY

Two follow-ups:

- aria2 peers now report progress (0..1) derived from the peer's piece bitfield
  (seeder => 1.0), matching qBittorrent. Previously the Peers list showed '—'
  for aria2 because the field was never populated.
- The server builder copied the whole repo via 'COPY . .', so a cmd/-only
  (downloader) change busted the layer and forced a full vite/tsup rebuild. Copy
  only the build inputs + final-stage sources (src, server, shared, public,
  index.html, vite.config.ts, tsconfig.json, migrations, scripts) so
  downloader-only pushes reuse the cached server image. Verified the builder
  stage still builds and produces dist/dist-server/migrations/entrypoint.

Dockerfile

@@ -11,7 +11,16 @@ RUN --mount=type=cache,target=/root/.local/share/pnpm/store \
     corepack enable \
  && pnpm install --frozen-lockfile
 
-COPY . .
+# Copy only what the JS build (vite + tsup) and the final server image consume,
+# so a cmd/-only change (the Go downloader) doesn't bust this layer and force a
+# full server rebuild. Keep in sync with the build inputs + the final-stage COPYs.
+COPY index.html vite.config.ts tsconfig.json ./
+COPY src ./src
+COPY server ./server
+COPY shared ./shared
+COPY public ./public
+COPY migrations ./migrations
+COPY scripts ./scripts
 # .git is excluded from the build context, so git describe cannot run here.
 # The release workflow passes the tag via APP_VERSION and the commit SHA via
 # APP_COMMIT; resolveAppVersion/resolveAppCommit read them.

cmd/internal/engine/aria2.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math/bits"
 	"net/http"
 	"net/url"
 	"os"
@@ -1026,6 +1027,34 @@ func aria2Leechers(peers []arigo.Peer) *int64 {
 	return &count
 }
 
+// aria2PeerProgress derives a peer's 0..1 completion from its piece bitfield
+// (aria2 has no direct percentage like qBittorrent). A seeder has every piece.
+// Spare bits past the last piece are zero, so dividing by the bitfield's bit
+// length only differs from the true piece count by sub-1% — fine for a bar.
+func aria2PeerProgress(peer arigo.Peer) *float64 {
+	if peer.Seeder {
+		full := 1.0
+		return &full
+	}
+	if peer.BitField == "" {
+		return nil
+	}
+	set := 0
+	for _, digit := range peer.BitField {
+		nibble, err := strconv.ParseUint(string(digit), 16, 8)
+		if err != nil {
+			return nil
+		}
+		set += bits.OnesCount8(uint8(nibble))
+	}
+	total := len(peer.BitField) * 4
+	if total == 0 {
+		return nil
+	}
+	progress := float64(set) / float64(total)
+	return &progress
+}
+
 func aria2Peers(peers []arigo.Peer, geoIP PeerGeoIPResolver) []client.DownloadTaskPeer {
 	out := make([]client.DownloadTaskPeer, 0, min(len(peers), 20))
 	for _, peer := range peers {
@@ -1036,6 +1065,7 @@ func aria2Peers(peers []arigo.Peer, geoIP PeerGeoIPResolver) []client.DownloadTa
 		up := int64(peer.UploadSpeed)
 		item := client.DownloadTaskPeer{
 			Address:     fmt.Sprintf("%s:%d", peer.IP, peer.Port),
+			Progress:    aria2PeerProgress(peer),
 			DownloadBps: &down,
 			UploadBps:   &up,
 		}

cmd/internal/engine/engine_test.go

@@ -201,6 +201,21 @@ func TestAria2SeedTimeMinutes(t *testing.T) {
 	}
 }
 
+func TestAria2PeerProgress(t *testing.T) {
+	if p := aria2PeerProgress(arigo.Peer{Seeder: true}); p == nil || *p != 1.0 {
+		t.Fatalf("expected seeder progress 1.0, got %v", p)
+	}
+	if p := aria2PeerProgress(arigo.Peer{BitField: "ff"}); p == nil || *p != 1.0 {
+		t.Fatalf("expected full bitfield 1.0, got %v", p)
+	}
+	if p := aria2PeerProgress(arigo.Peer{BitField: "f0"}); p == nil || *p != 0.5 {
+		t.Fatalf("expected half bitfield 0.5, got %v", p)
+	}
+	if p := aria2PeerProgress(arigo.Peer{BitField: ""}); p != nil {
+		t.Fatalf("expected nil progress for empty bitfield, got %v", p)
+	}
+}
+
 func TestQBittorrentStartArgsWritesManagedListenPort(t *testing.T) {
 	stateDir := t.TempDir()
 	downloadDir := t.TempDir()