feat(cast): add server-managed Google Cast playback (#34)

Add server-side Google Cast discovery and sessions, signed cast media URLs, browser device picker integration, and deployment documentation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -13,6 +13,8 @@
         "@ffprobe-installer/ffprobe": "^2.1.2",
         "bcryptjs": "^3.0.3",
         "better-sqlite3": "^12.5.0",
+        "bonjour-service": "^1.4.4",
+        "chromecast-client": "^1.0.1",
         "dotenv": "^17.2.3",
         "express": "^4.18.2",
         "express-session": "^1.18.2",
@@ -27,6 +29,11 @@
     "optionalDependencies": {
         "ffmpeg-static": "^5.3.0"
     },
+    "overrides": {
+        "castv2": {
+            "protobufjs": "^7.6.3"
+        }
+    },
     "engines": {
         "node": ">=18.0.0"
     },