feat: add Android app and end-to-end encrypted library sync

Introduce the Kotlin/Compose Android companion app and extend the TMDB
proxy into a combo server that synchronizes the whole library as an
encrypted snapshot, with all three components (Go server, Android,
desktop) sharing one zero-knowledge crypto envelope and JSON schema.

- server: add PostgreSQL-backed /sync/library (GET/PUT) with optimistic
  concurrency (revision + 409 on conflict); sync stays disabled unless
  DATABASE_URL is set. Add docker-compose with Postgres.
- desktop: add libsodium CryptoEnvelope, LibrarySerializer and SyncClient;
  storage-mode and passphrase settings; a "Sync now" toolbar action with
  conflict resolution.
- android: Room-backed library with local/cloud storage modes, encrypted
  sync client, and settings UI. The proxy server (URL + token) can be
  configured as a TMDB proxy even in local-only mode.

Crypto is identical across platforms: Argon2id (libsodium crypto_pwhash,
INTERACTIVE) + XChaCha20-Poly1305 in a versioned "UMTS" envelope, so a
snapshot encrypted on one client decrypts on the other.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/sync.go

@@ -0,0 +1,123 @@
+package main
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// Maximum accepted snapshot size (encrypted). Generous for a personal library.
+const maxSyncPayload = 16 << 20 // 16 MiB
+
+type syncGetResponse struct {
+	Revision  int64  `json:"revision"`
+	UpdatedAt string `json:"updatedAt"`
+	Payload   string `json:"payload"` // base64 ciphertext
+}
+
+type syncPutRequest struct {
+	BaseRevision int64  `json:"baseRevision"`
+	Payload      string `json:"payload"` // base64 ciphertext
+}
+
+type syncPutResponse struct {
+	Revision int64 `json:"revision"`
+}
+
+type syncConflictResponse struct {
+	Error    string `json:"error"`
+	Revision int64  `json:"revision"`
+}
+
+// authSubject validates the Bearer token and returns the user's subject/email.
+func (s *server) authSubject(r *http.Request) (sub, email string, ok bool) {
+	bearer := strings.TrimSpace(strings.TrimPrefix(r.Header.Get("Authorization"), "Bearer"))
+	claims, err := verifyToken(s.cfg.SessionKey, bearer)
+	if err != nil {
+		return "", "", false
+	}
+	return claims.Subject, claims.Email, true
+}
+
+// handleSyncLibrary dispatches GET/PUT for the encrypted library snapshot.
+func (s *server) handleSyncLibrary(w http.ResponseWriter, r *http.Request) {
+	if s.store == nil {
+		http.Error(w, "sync is not configured on this server", http.StatusNotImplemented)
+		return
+	}
+	sub, email, ok := s.authSubject(r)
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	switch r.Method {
+	case http.MethodGet:
+		s.handleSyncGet(w, r, sub)
+	case http.MethodPut:
+		s.handleSyncPut(w, r, sub, email)
+	default:
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+	}
+}
+
+func (s *server) handleSyncGet(w http.ResponseWriter, r *http.Request, sub string) {
+	blob, err := s.store.getBlob(r.Context(), sub)
+	if err != nil {
+		http.Error(w, "storage error", http.StatusInternalServerError)
+		return
+	}
+	if blob == nil {
+		http.Error(w, "no snapshot yet", http.StatusNotFound)
+		return
+	}
+	writeJSON(w, http.StatusOK, syncGetResponse{
+		Revision:  blob.Revision,
+		UpdatedAt: blob.UpdatedAt.UTC().Format(time.RFC3339),
+		Payload:   base64.StdEncoding.EncodeToString(blob.Payload),
+	})
+}
+
+func (s *server) handleSyncPut(w http.ResponseWriter, r *http.Request, sub, email string) {
+	var req syncPutRequest
+	body := http.MaxBytesReader(w, r.Body, maxSyncPayload+(1<<16))
+	if err := json.NewDecoder(body).Decode(&req); err != nil {
+		http.Error(w, "bad request body", http.StatusBadRequest)
+		return
+	}
+	payload, err := base64.StdEncoding.DecodeString(req.Payload)
+	if err != nil {
+		http.Error(w, "payload is not valid base64", http.StatusBadRequest)
+		return
+	}
+	if len(payload) == 0 {
+		http.Error(w, "empty payload", http.StatusBadRequest)
+		return
+	}
+	if len(payload) > maxSyncPayload {
+		http.Error(w, "payload too large", http.StatusRequestEntityTooLarge)
+		return
+	}
+
+	next, err := s.store.putBlob(r.Context(), sub, email, req.BaseRevision, payload)
+	if err == errRevisionConflict {
+		writeJSON(w, http.StatusConflict, syncConflictResponse{
+			Error:    "revision conflict",
+			Revision: next,
+		})
+		return
+	}
+	if err != nil {
+		http.Error(w, "storage error", http.StatusInternalServerError)
+		return
+	}
+	writeJSON(w, http.StatusOK, syncPutResponse{Revision: next})
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v)
+}