feat: bitwise RBAC engine with channel-level permission overrides

Replace string-based role checks (role === 'admin') with a bitwise BigInt
permission system. Adds computePermissions() resolution engine following
Discord's model: @everyone base → role union → admin shortcut → channel
overrides (role deny/allow → member deny/allow). Ready payload now filters
channels by VIEW_CHANNEL and attaches per-user myPermissions to each
server and channel. Includes channel_overrides table, @everyone role
auto-creation, migration for existing servers, and override CRUD API.

packages/shared/package.json

@@ -6,7 +6,9 @@
   "main": "./src/types.ts",
   "types": "./src/types.ts",
   "exports": {
-    ".": "./src/types.ts"
+    ".": "./src/types.ts",
+    "./src/permissions": "./src/permissions.ts",
+    "./src/permissions.js": "./src/permissions.ts"
   },
   "scripts": {
     "build": "tsc",