## Why
With several PuppetDBs behind one endpoint, consumers cannot tell which backend a node's data came from.
## How
- Add a synthetic `pdbmux_source` fact per certname on `/facts`, valued with the backend that won the facts merge, and stamp the same key on merged `/nodes` records.
- Emit all four fact keys including `environment`, which clients index directly.
- Skip injection for top-level `extract` queries, so `count()` and other aggregates keep the backends' own numbers, and for `/facts` queries constraining `name`; a `name` filter inside an `in` subquery still injects.
- Replace, never duplicate, an upstream fact of the configured name.
- Configure via `source_fact` / `source_fact_enabled` (`PDBMUX_SOURCE_FACT`, `PDBMUX_SOURCE_FACT_ENABLED`), defaulting to `pdbmux_source` enabled.
