build(setup): prove the venv actually works instead of trusting pip's exit code

`make setup` created the venv and ran pip install, then exited 0 whether or
not the result was usable — exactly D-24's failure shape, and the one this
repo hit hardest: on 2026-08-09 there was no venv at all here while CLAUDE.md
documented .venv/bin/python -m pytest as the way to run tests, and nothing in
setup would have caught that state before a person did.

Add a `pytest tests/ --collect-only -q` step at the end. It imports every
test module and everything each one pulls in from src/, without running a
single test, and fails the target on a broken interpreter or a broken
dependency graph alike.

Verified directly:
- absent interpreter -> exit 127, target halts
- apscheduler (declared, imported by src/main.py) uninstalled -> collect-only
  exits 4 with ModuleNotFoundError, target halts
- make setup afterwards reinstalls it and collect-only exits 0, 189 tests
  collected, same count as before the mutation
- idempotent: a warm rerun on an already-correct venv changes nothing and
  collects the same 189 tests, just faster (7s vs 30s cold)

T-47 (workspace).

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

Makefile

@@ -17,9 +17,18 @@ help: ## Show this help
 	  | awk 'BEGIN{FS=":.*?## "}{printf "  \033[36m%-14s\033[0m %s\n", $$1, $$2}'
 
 .PHONY: setup
-setup: ## Create the venv and install the test extra
+setup: ## Create the venv, install the test extra, and prove it actually works
 	$(PYTHON) -m venv .venv
 	$(VENV)/bin/pip install -e ".[test]"
+	@# Exit 0 from `pip install` is not evidence (D-24) — pip reports success even
+	@# when the result is unusable. Prove the environment works instead of trusting
+	@# the install step: `--collect-only` imports every test module and therefore
+	@# every src module each one pulls in, which is exactly the failure mode this
+	@# target exists to catch (T-47 — this repo had no venv at all on 2026-08-09,
+	@# and the documented test command could not work). It runs zero tests, so it
+	@# stays cheap, and unlike a bare `import src.main` it exercises the tests/
+	@# tree too, not just the package.
+	$(VENV)/bin/python -m pytest tests/ --collect-only -q
 
 .PHONY: test
 test: ## Run the test suite