feat(api): determine if docker container has update

[pujitm]

• Add a new utility class, AsyncMutex in unraid-shared -> processing.ts, for ergonomically de-duplicating async operations.
• Add an @OmitIf decorator for omitting graphql queries, mutations, or field resolvers from the runtime graphql schema.
• Add feature-flagging system
  • FeatureFlags export from consts.ts
  • @UseFeatureFlag decorator built upon OmitIf
  • checkFeatureFlag for constructing & throwing a ForbiddenError if the given feature flag evaluates to false.

Summary by CodeRabbit

• New Features

  • Expose disk spinning state, per-container "update available" and "rebuild ready" indicators, a structured per-container update-status list (enum: UP_TO_DATE, UPDATE_AVAILABLE, REBUILD_READY, UNKNOWN), and a mutation to refresh Docker digests. Periodic and post-startup digest refreshes added (feature-flag gated).

• Chores

  • Centralized scheduler setup and cron wiring; build now bundles a PHP wrapper asset; added feature-flag env var and local keys .gitignore entry.

• Documentation

  • Added developer guide for feature flags.

• Tests

  • New test suites for concurrency mutex, parser, decorators, Docker config, and Docker status parsing.