chore(runner): raise DRONE_RUNNER_CAPACITY 1 -> 2 for parallel recipe CI

Lets two recipes be tested in parallel (operator request — immich + plausible under
active dev at once). Safe on the current node: measured a full immich CI stack at
~1GiB with multiple GiB free on the 7.6GiB cpx22, and the janitor is already
age-based + run-app-scoped so it never reaps a concurrent in-flight run. Updates the
stale '28GiB node' comment. Revert to 1 if OOM/IO contention shows up.

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

nix/modules/drone-runner.nix

@@ -9,13 +9,18 @@
 let
   # MAX_TESTS (plan §4.2/§4.3 resource safety): max CI builds the exec runner runs at once. Drone
   # queues the rest in its native pending-build queue (no custom queue). THE concurrency cap that
-  # bounds how many test apps can be live at once — kept LOW (1) on this single 28GiB node since
-  # recipes are heavy (immich/matrix large volumes). With capacity=1 there is never a concurrent
-  # in-flight run, so the run-start janitor can safely reap *any* orphan (a SIGKILL'd build runs no
-  # teardown) and the "at most MAX_TESTS apps live" bound holds exactly. Raise to 2 only if the node
-  # is shown to handle two light recipes at once (then the janitor MUST stay age-based to avoid
-  # reaping a concurrent run — see DECISIONS.md "Resource safety").
-  maxTests = "1";
+  # bounds how many test apps can be live at once.
+  #
+  # Raised to 2 (operator request 2026-06-09) so two recipes can be tested in parallel (e.g. immich
+  # and plausible under active development at once). Verified safe on the current node (Hetzner cpx22,
+  # ~7.6 GiB / 4 vCPU — NOTE: smaller than the original 28 GiB this was written for): a full immich CI
+  # stack measured ~1 GiB (server+ML+pg+redis) with multiple GiB free, so two concurrent recipes fit.
+  # The concurrency PRECONDITION holds: the run-start janitor is age-based (default 2h) + run-app-name
+  # scoped, so it never reaps a concurrent in-flight run (harness.lifecycle.janitor). TRADE-OFF: with
+  # capacity>1 a SIGKILL'd build (no teardown) leaves an orphan the run-start sweep can't reap
+  # immediately (it might be a live run) — bounded instead by the 2h janitor + the /upgrade-all
+  # start/end reap + sweep-orphans. Revert to "1" if OOM / disk-I/O contention is observed under load.
+  maxTests = "2";
 in
 {
   # Drone ships under the Polyform Small Business license (nixpkgs marks it unfree);