fix(fitness): eliminate cross-user data access (#4)

- All user_id query params now enforced to authenticated user's own ID
- /api/users restricted to return only current user (no user enumeration)
- Wildcard CORS headers removed (service is internal-only via gateway)
- Covers: entries, totals, goals, templates, favorites, goal setting

Closes #4

services/fitness/server.py

@@ -1977,7 +1977,7 @@ class CalorieHandler(BaseHTTPRequestHandler):
         self.send_response(status)
         self.send_header('Content-Type', 'application/json')
         self.send_header('Content-Length', len(body))
-        self.send_header('Access-Control-Allow-Origin', '*')
+        # CORS removed — service is internal only
         self.send_header('Access-Control-Allow-Headers', 'Content-Type, Authorization, X-API-Key, X-Telegram-User-Id')
         self.send_header('Access-Control-Allow-Methods', 'GET, POST, PATCH, PUT, DELETE, OPTIONS')
         self.end_headers()
@@ -2002,7 +2002,7 @@ class CalorieHandler(BaseHTTPRequestHandler):
     def do_OPTIONS(self):
         """Handle CORS preflight."""
         self.send_response(204)
-        self.send_header('Access-Control-Allow-Origin', '*')
+        # CORS removed — service is internal only
         self.send_header('Access-Control-Allow-Headers', 'Content-Type, Authorization, X-API-Key, X-Telegram-User-Id')
         self.send_header('Access-Control-Allow-Methods', 'GET, POST, PATCH, PUT, DELETE, OPTIONS')
         self.end_headers()
@@ -2116,21 +2116,21 @@ class CalorieHandler(BaseHTTPRequestHandler):
         # GET /api/entries?date=...&user_id=...
         if path == '/api/entries':
             entry_date = params.get('date', [date.today().isoformat()])[0]
-            target_user = params.get('user_id', [user['id']])[0]
+            target_user = user['id']  # enforced: no cross-user access
             entries = get_entries_by_date(target_user, entry_date)
             return self._send_json(entries)
 
         # GET /api/entries/totals?date=...
         if path == '/api/entries/totals':
             entry_date = params.get('date', [date.today().isoformat()])[0]
-            target_user = params.get('user_id', [user['id']])[0]
+            target_user = user['id']  # enforced: no cross-user access
             totals = get_daily_totals(target_user, entry_date)
             return self._send_json(totals)
 
         # GET /api/goals/for-date?date=...&user_id=...
         if path == '/api/goals/for-date':
             for_date = params.get('date', [date.today().isoformat()])[0]
-            target_user = params.get('user_id', [user['id']])[0]
+            target_user = user['id']  # enforced: no cross-user access
             goal = get_goals_for_date(target_user, for_date)
             if goal:
                 return self._send_json(goal)
@@ -2138,7 +2138,7 @@ class CalorieHandler(BaseHTTPRequestHandler):
 
         # GET /api/goals?user_id=...
         if path == '/api/goals':
-            target_user = params.get('user_id', [user['id']])[0]
+            target_user = user['id']  # enforced: no cross-user access
             conn = get_db()
             rows = conn.execute(
                 "SELECT * FROM goals WHERE user_id = ? ORDER BY start_date DESC",
@@ -2190,12 +2190,9 @@ class CalorieHandler(BaseHTTPRequestHandler):
             conn.close()
             return self._send_json([dict(r) for r in rows])
 
-        # GET /api/users (list all users — for switching view)
+        # GET /api/users — restricted: only returns the current user
         if path == '/api/users':
-            conn = get_db()
+            return self._send_json([{"id": user["id"], "username": user["username"], "display_name": user["display_name"]}])
-            rows = conn.execute("SELECT id, username, display_name FROM users").fetchall()
-            conn.close()
-            return self._send_json([dict(r) for r in rows])
 
         self._send_json({'error': 'Not found'}, 404)
 
@@ -2222,7 +2219,7 @@ class CalorieHandler(BaseHTTPRequestHandler):
             self.send_response(200)
             self.send_header('Content-Type', 'application/json')
             self.send_header('Set-Cookie', f'session={token}; Path=/; HttpOnly; SameSite=Lax; Max-Age=2592000')
-            self.send_header('Access-Control-Allow-Origin', '*')
+            # CORS removed — service is internal only
             body = json.dumps({'token': token, 'user': {
                 'id': user['id'], 'username': user['username'], 'display_name': user['display_name']
             }}).encode()
@@ -2583,7 +2580,7 @@ class CalorieHandler(BaseHTTPRequestHandler):
         # PUT /api/goals
         if path == '/api/goals':
             data = self._read_body()
-            target_user = data.get('user_id', user['id'])
+            target_user = user['id']  # enforced: no cross-user access
             start_date = data.get('start_date', date.today().isoformat())
 
             conn = get_db()