fix: scope marks, labels, and subjects to the authenticated user

Marks (ratings/color labels) were stored without a user column — every
user saw every other user's marks. Labels and subjects from PhotoPrism's
global endpoints leaked across users because those endpoints ignore
BasePath ACL.

Sidecar:
- Add UserName as composite primary key on Mark (photo_uid, user_name)
- Replace validateSession with resolveSession that fetches the user
  identity from PhotoPrism's session endpoint
- Filter all mark queries by user_name

Frontend:
- Filter listLabels/listSubjects through a BasePath-aware existence
  check — each label/subject is kept only if the user has at least one
  matching photo (single count=1 probe per item, batched at concurrency 8)
- Skip filtering for admin users with empty BasePath (single-user compat)

Also documents USER_BASEPATHS in .env.example — the env var that drives
per-user library isolation via PhotoPrism's auth_users.base_path.

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

sidecar/pp.go

@@ -85,6 +85,37 @@ func (c *ppClient) call(ctx context.Context, method, urlPath, token string, body
 	}, nil
 }
 
+// ppSessionUser is the subset of PhotoPrism's session response we need.
+type ppSessionUser struct {
+	UserName string `json:"UserName"`
+	UserUID  string `json:"UserUID"`
+	BasePath string `json:"BasePath"`
+}
+
+type ppSessionResponse struct {
+	User ppSessionUser `json:"user"`
+}
+
+// resolveSession validates the token AND returns the authenticated user.
+// Returns nil when the token is invalid or the response can't be parsed.
+func (c *ppClient) resolveSession(ctx context.Context, token string) *ppSessionUser {
+	if token == "" {
+		return nil
+	}
+	r, err := c.call(ctx, http.MethodGet, "/api/v1/session/"+token, token, nil)
+	if err != nil || !r.OK {
+		return nil
+	}
+	var resp ppSessionResponse
+	if err := json.Unmarshal(r.Body, &resp); err != nil {
+		return nil
+	}
+	if resp.User.UserName == "" {
+		return nil
+	}
+	return &resp.User
+}
+
 // validateSession is the cheapest probe that the supplied token is live:
 // list one photo. 401 → bad/expired token. We never read the payload.
 func (c *ppClient) validateSession(ctx context.Context, token string) bool {