feat(api): environment cloning and flag import/export

[trinhvandat]

Context (P2 — product)

Setting up a new environment or promoting flag config between environments is manual, flag by flag.

Scope

• POST /api/v1/environments/{id}/clone — new environment copying all flag states (new API key generated)
• GET /api/v1/environments/{id}/export — JSON snapshot of flags + states (schema-versioned envelope)
• POST /api/v1/environments/{id}/import — apply snapshot with dry-run mode and conflict strategy (skip/overwrite)
• Permission: ADMIN+ on the org

Acceptance criteria

• Clone produces identical flag states with a fresh API key
• Export→import round-trip is lossless (test)
• Import dry-run reports the change set without applying
• Import validates the schema version and rejects unknown versions cleanly