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/web/src/utils/permissions.ts

@@ -0,0 +1,11 @@
+// Frontend permission helpers — wraps shared permission constants.
+// Always works with string representations (never raw bigint in state).
+
+export {
+  PermissionBits,
+  ALL_PERMISSIONS,
+  DEFAULT_EVERYONE_PERMISSIONS,
+  hasPermissionBit,
+  permissionsToString,
+  stringToPermissions,
+} from '@opencord/shared/src/permissions';