Refuse an /events aggregate that also asks for distinct_resources

distinct_resources sends /events to openvoxdb's legacy compiler, which has no
function or group_by, so every backend failed and the client saw a generic 502
instead of the reason.

- Refuse an aggregate carrying a truthy distinct_resources with 400, before any fan-out
- Read the param the way openvoxdb does, so any capitalisation of "true" counts
- Leave non-aggregate distinct_resources queries and every other route alone
- Document the refusal

README.md

@@ -160,6 +160,13 @@ paths add two more headers `pdbmux` sets itself, `X-Cache` and `Age` — see
   - A `/reports` or `/events` query with no `function` column is a projection of
     real records, not an aggregate, and stays on the union path — so an event
     stored identically in two backends is still served once.
+  - `distinct_resources=true` on an `/events` `extract` with a `function` column
+    is refused with **400** naming the incompatibility, before any fan-out.
+    openvoxdb answers the distinct-resources form of `/events` from its legacy
+    compiler, which supports neither `function` nor `group_by`, so every backend
+    fails and the mistake would otherwise surface as a `502`. A
+    `distinct_resources` query with no `function` column is untouched and still
+    fans out; no other endpoint `pdbmux` combines honours the parameter.
   - `include_total=true` on a combined endpoint reports the **merged** row count,
     not the sum of the backends' `X-Records`, since shared keys collapse.