fix(rust): read .env like python-dotenv; dry run in the service environment

dotenvy aborted on the production .env (a quote in an unquoted value makes it
read to the end of the file) and echoed the offending line, password
included. envfile.rs ports python-dotenv 1.1.1 (parser + ${VAR} interpolation,
no override): unparsable statements are skipped with a warning that names only
the line number. Verified against python-dotenv on 614 generated inputs
(testdata/). The .env is loaded before the tokio runtime starts.

install_collector.sh now runs the dry run through systemd-run with the unit's
User, WorkingDirectory and EnvironmentFile, i.e. exactly what the service sees.

rust/README.md

@@ -25,7 +25,10 @@ ping-collector check-db             # connect, create missing tables, row counts
 ```
 
 Configuration is the Python app's `.env` (read from the working directory, or
-`--env-file`); real environment variables win. Used keys:
+`--env-file`) with python-dotenv's rules, verified on 614 generated inputs
+(`collector/testdata/`): an unparsable line is skipped with a warning giving
+only its line number, never its value. Variables already set (e.g. by systemd's
+`EnvironmentFile=`) win. Used keys:
 `POSTGRES_CONNECTION_STRING` (or `POSTGRES_HOST/PORT/DB/USER/PASSWORD`),
 `RID_POLL_OFFSET_MINUTES` (5), `RID_RETRY_MINUTES` (10),
 `GAP_FILL_DAYS_ON_START` (14), `ENABLE_HII_COLLECTION`,