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Build me a feature flag service that can never take my application down.

STACK
- Node with Fastify, SQLite via better-sqlite3
- Server-rendered admin, no frontend framework

FLAGS
- Key, description, a default value, and typed variations: boolean, string, number, JSON

TARGETING
- Rules evaluated in order: individual user keys, attribute matches, then a percentage rollout
- Bucketed by a stable hash of flag key plus user key, so a user's bucket never moves when an unrelated flag changes

THE RULE THAT MATTERS MOST
- One endpoint returns every flag for a user context in a single call
- The client caches the payload in memory and refreshes on an interval
- When the service is unreachable it falls back to a compiled-in default — the flag service must never be able to take the application down

AUDIT
- Every change appended with previous value, new value, actor and UTC timestamp

OPERATIONS
- A kill-switch view listing every enabled flag with a one-click disable
- An SSE stream so a change propagates in seconds rather than at the next poll

TESTS
- Prove the same user lands in the same bucket across restarts
- Prove the client keeps serving defaults when the service is stopped mid-request

Include a README on the hashing scheme, the cache interval and the fallback behaviour.