* Fix: Load extensions in server.py for multi-worker deployments
When running with multiple workers (--workers 2), uvicorn uses
`hindsight_api.server:app` import string instead of passing an app
object. The server.py module was not loading tenant/operation validator
extensions, causing authentication bypass in production.
This fix:
- Adds extension loading to server.py matching main.py behavior
- Sets extension context on tenant extension for schema provisioning
- Adds comprehensive unit tests for server.py extension loading
The tests specifically verify:
- TENANT extension is loaded when HINDSIGHT_API_TENANT_EXTENSION is set
- OPERATION_VALIDATOR is loaded when configured
- Extensions are passed to MemoryEngine constructor
- Extension context is set on tenant extension
- Server works correctly without extensions configured
* Add unit tests for main.py extension loading (single-worker path)
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