test: provision MongoDB 8 with Testcontainers

docs/journals/260721-0923-testcontainers-mongodb-integration.md

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+# Testcontainers MongoDB Integration Journal
+
+## Context
+
+MongoDB integration tests previously depended on contributors manually starting
+a database and setting `MONGODB_TEST_URL`. The suite now provisions its own
+MongoDB 8 instances when Docker is available.
+
+## What Changed
+
+- Added Testcontainers Go and MongoDB module v0.43.0.
+- Added a shared lazy test manager that starts one `mongo:8` container per
+  `storage`, `lol`, and `stats` package and terminates it after that package's
+  suite.
+- Preserved `MONGODB_TEST_URL` as an external-database override.
+- Docker-unavailable environments emit an explicit warning and skip MongoDB
+  tests; a regression test verifies both the warning and skip behavior.
+- Once the Docker provider is healthy, container startup and connection-string
+  failures are fatal. Cleanup failures also make an otherwise passing suite
+  fail.
+- Updated README and CI guidance so normal Go test commands exercise
+  Testcontainers automatically.
+
+## Reflection
+
+Package-scoped lazy containers balance isolation and startup cost: packages do
+not share database processes, while tests inside a package avoid repeatedly
+starting MongoDB. Distinguishing an absent Docker daemon from a broken healthy
+provider keeps local no-Docker runs usable without hiding real infrastructure
+regressions.
+
+## Decisions
+
+- Pin tests and local setup guidance to MongoDB 8.
+- Keep explicit external MongoDB support for constrained or pre-provisioned
+  environments.
+- Treat lifecycle failures as test failures whenever Docker is available.
+
+## Verification
+
+- Passed: `go test ./...`
+- Passed: `go vet ./...`
+- Passed: `go build ./...`
+- Passed: `golangci-lint run`
+- Passed: `go mod tidy` with no residual module diff.
+- Three real MongoDB containers ran, one for each integration-test package, and
+  all three were cleaned up successfully.
+
+## Next Steps
+
+- Monitor CI duration and container startup reliability after rollout.