chore: feature flag build system for Mini

[ng]

Context

Add build-time feature flag so Mini code is excluded from standard pod builds.

Implementation

• Environment variable: ENABLE_MINI=true
• Conditional dynamic imports for Mini services, routes, and PubNub client
• When disabled: zero additional dependencies bundled, no PubNub SDK, no Mini routes
• CI matrix produces two TARs: sleepypod-core.tar.gz and sleepypod-core-mini.tar.gz

Files affected

• next.config.ts — conditional webpack alias or define
• src/server/routers/index.ts — conditional Mini router merge
• src/services/mini/ — entire directory tree-shaken when disabled
• package.json — PubNub SDK as optional dependency
• CI workflow — matrix build with/without ENABLE_MINI

Parent

Epic: #405