refactor(flags): remove maintenance-mode check (operator-native now)

[coopbri]

Summary

Removes the dead Elysia maintenanceMiddleware and its Unleash-backed feature-flag plumbing. Unleash was decommissioned and maintenance mode is now operator-native, so this code path is dead.

Changes

• Delete src/lib/middleware/maintenance.ts
• Remove the middleware import and .use(maintenanceMiddleware) from src/server.ts
• Drop the flags / createFlagProvider provider (plus FLAGS_API_HOST / FLAGS_CLIENT_KEY imports) from src/lib/providers/index.ts (other providers retained)
• Remove FLAGS_API_HOST / FLAGS_CLIENT_KEY and the "FLAGS not set" startup warning from src/lib/config/env.config.ts
• Remove FLAGS_* lines from .env.production, .env.local.template (and local .env.local)

@omnidotdev/providers is retained (still used by authz, billing, events, storage, and GraphQL plugins).

Verification

• grep -rniE "maintenance|createFlagProvider|FLAGS_" src clean
• bunx tsc --noEmit passes
• bun run check (biome) passes (only pre-existing warnings)
• bun run knip no new issues
• bun run build passes