ci: give the backend job headroom over its observed tail (#1108)

The backend job normally finishes in about 3 minutes — the last eight
runs on main were 2.6 to 3.4 — but it is the only one that boots
Postgres and runs the full integration suite, so it is the only one
exposed to runner contention. The observed spread has reached 9.2
minutes against a 10-minute cap, and release PR #1088 was cancelled at
10.3 with every test in the log passing and jest still running.

That failure mode is expensive out of proportion to how often it
happens: a cancelled job is a red X on a branch that is actually green,
so it costs a diagnosis and a re-run each time, and it lands on release
PRs because those are the ones that run when everything else does.

The cap is a runaway guard rather than a performance budget, so 20 buys
real headroom over the worst run seen while still killing a genuinely
hung suite well inside the hour GitHub would otherwise allow.

frontend and ml keep 10: they finish in seconds and have never been
close.

Co-authored-by: Paul Nothaft <paul@MacStudio-von-Paul.local>

.github/workflows/tests.yml

@@ -28,7 +28,18 @@ permissions:
 jobs:
   backend:
     runs-on: ubuntu-latest
-    timeout-minutes: 10
+    # 20, not 10. This job normally finishes in ~3 minutes, but it is the only
+    # one that boots Postgres and runs the full integration suite, so it is the
+    # only one exposed to runner contention — observed spread has reached 9.2
+    # minutes, and a release PR (#1088) was cancelled at 10.3 with every test
+    # passing and jest still running. A cancelled job reads as a red X on a
+    # green branch, which costs a re-run and a diagnosis every time it happens.
+    #
+    # The cap is a runaway guard, not a performance budget; 20 leaves real
+    # headroom over the worst observed run while still killing a hung suite
+    # well inside the hour GitHub would otherwise allow. frontend and ml keep
+    # 10 — they take seconds and have never come close.
+    timeout-minutes: 20
 
     # The .picpeak restore suites gate their real-Postgres cases behind
     # PICPEAK_PG_TEST_URL and `describe.skip` themselves out when it is