Stop system GeoIP files from shadowing the bundled IPinfo database (#813)

* Stop system GeoIP files from shadowing the bundled IPinfo database

_get_ip_database_path() searched well-known system paths (including
/usr/share/GeoIP/GeoLite2-Country.mmdb and CWD-relative names) before
the database parsedmarc manages, so on any host with a distro GeoIP
package installed every lookup silently used a country-only — and
often years-old — database instead of the bundled IPinfo Lite one.
That disabled ASN enrichment entirely (asn/as_name/as_domain were None
for every IP) and with it the ASN-fallback path into the reverse-DNS
map, with no signal beyond a generic "IP database is more than a
month old" warning. Verified live on a Fedora host whose distro
GeoLite2-Country.mmdb dated to December 2019.

New precedence: explicit ip_db_path -> _IP_DB_PATH selected by
load_ip_db() (downloaded/cached/bundled) -> the bundled copy -> system
paths as a true last resort (only consulted when the bundled data
file is missing). The selected file is logged at debug level so a
--debug run shows which database answered.

The automatic system-path pickup was documented behavior, so
installation.md now tells MaxMind GeoLite2 users to set ip_db_path
explicitly, with a migration note.

Both new regression tests reproduce the shadowing portably via a decoy
CWD GeoLite2-Country.mmdb (the fallback list includes relative names),
and fail on the unfixed code (verified by stashing the source change).

Fixes https://github.com/domainaware/parsedmarc/issues/810.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Address review: accurate fallback comment, dedup selection log, cover fallback tiers

- Correct the nothing-found-anywhere comment: the os.stat() age check
  raises FileNotFoundError before the caller's open_database() would.
- Log "Using IP database at ..." only when the selected path changes
  instead of on every uncached IP lookup, so --debug runs over large
  batches aren't flooded; tracked via _LAST_LOGGED_IP_DB_PATH, reset in
  the test fixture for order-independence.
- Cover the previously untested branches of _get_ip_database_path:
  system-path fallback when the bundled database is missing, the
  FileNotFoundError when nothing exists anywhere, the stale-database
  warning, and the log-once-per-path behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: MISAPOR LAB <misapor@lab.misapor.pl>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Sean Whalen <44679+seanthegeek@users.noreply.github.com>

docs/source/installation.md

@@ -119,12 +119,28 @@ This installs the `msgconvert` script to `/usr/local/bin/msgconvert`.
 
 ## Using MaxMind GeoLite2 (optional)
 
-`parsedmarc` will pick up the [MaxMind GeoLite2 Country database] if
-it is installed at one of the standard system paths (e.g.
-`/usr/share/GeoIP/GeoLite2-Country.mmdb`,
-`/var/lib/GeoIP/GeoLite2-Country.mmdb`, or the equivalent location on
-Windows). **Use this only if you specifically prefer MaxMind data over
-the bundled IPinfo Lite database — most users do not need it.**
+To use the [MaxMind GeoLite2 Country database] instead of the bundled
+IPinfo Lite database, point the `ip_db_path` option at it explicitly:
+
+```ini
+[general]
+ip_db_path = /usr/share/GeoIP/GeoLite2-Country.mmdb
+```
+
+**Use this only if you specifically prefer MaxMind data over the
+bundled IPinfo Lite database — most users do not need it.** Country
+databases like GeoLite2 carry no ASN data, so source attribution for
+IP addresses without reverse DNS is reduced when one is used.
+
+:::{note}
+parsedmarc no longer picks up a GeoLite2/DBIP database from standard
+system paths (e.g. `/usr/share/GeoIP/GeoLite2-Country.mmdb`)
+automatically — a GeoIP file installed by an unrelated distro package
+would silently override the bundled database and disable ASN
+enrichment. System paths are now only consulted as a last resort when
+the bundled database is missing. If you previously relied on the
+automatic pickup, set `ip_db_path` as shown above.
+:::
 
 Install [geoipupdate] for your platform: