fix(telemetry): FILTER must bind to the aggregate inside extract(), not outside

`extract(epoch from max(...)) filter (where ...)` is a syntax error — FILTER
attaches to extract() (not an aggregate). Move it inside: extract(epoch from
max(...) filter (where ...)). SAMPLE_SQL failed on every project's sampler tick.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: valvesss

control-plane/src/telemetry/sampler.ts

@@ -27,10 +27,10 @@ select
                     or state = 'idle in transaction (aborted)')::int as idle_in_tx,
   count(*) filter (where wait_event_type is not null and state = 'active')::int as waiting,
   count(*) filter (where wait_event_type = 'Lock')::int as blocked,
-  coalesce(extract(epoch from max(now() - xact_start))
-             filter (where state <> 'idle' and xact_start is not null), 0)::float as longest_tx_secs,
-  coalesce(extract(epoch from max(now() - state_change))
-             filter (where state = 'idle in transaction'), 0)::float as oldest_idle_in_tx_secs,
+  coalesce(extract(epoch from max(now() - xact_start)
+             filter (where state <> 'idle' and xact_start is not null)), 0)::float as longest_tx_secs,
+  coalesce(extract(epoch from max(now() - state_change)
+             filter (where state = 'idle in transaction')), 0)::float as oldest_idle_in_tx_secs,
   pg_database_size(current_database())::bigint as db_size
 from pg_stat_activity
 where datname = current_database()