docs: run tests with an in-memory DATABASE_URL

The docs said tests need BASE_PATH in .env. The full suite (lib, bin and
integration tests) passes with no .env and no BASE_PATH; the only thing
it needs is DATABASE_URL, and ":memory:" works. Without it, DB-backed
tests such as reels::tests::reel_capture_preserves_owner_defaults panic
with "DATABASE_URL must be set".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -243,3 +243,10 @@ unset.
   populated retroactively) [default: `2000`]. Errors don't burn the cap;
   only successful hashes count.
 
+## Testing
+`DATABASE_URL` is the only environment variable the test suite needs, and an
+in-memory SQLite database works, so no `.env` is required:
+
+```bash
+DATABASE_URL=":memory:" cargo test
+```