Look clients up in the IPinfo Lite file with SWWAF_LOOKUP_SOURCE=file (closes #22)

SWWAF_LOOKUP_SOURCE=file looks every client up in the file
SWWAF_LOOKUP_DB_PATH names, without GeoJS. file without the path, the
path with another source, or a file that cannot be read stops the start.
The file is read whole into memory, so overwriting it in place cannot
disturb a lookup, and read again 2 seconds after its last change; a
replacement that cannot be read is logged, counted and sent as a
file_error alert, and the old one stays in use. Metrics give when it
was read and the failed reads. Tests write their databases through
internal/lookup/lookuptest.

Deviation: go.mod and go.sum written by hand; go runs only through make.
Judgement call: the 2-second wait, as the rule files have.

Model: opus-5-5

go.mod

@@ -5,6 +5,8 @@ go 1.26.0
 require (
 	github.com/fsnotify/fsnotify v1.10.1
 	github.com/hashicorp/golang-lru/v2 v2.0.7
+	github.com/maxmind/mmdbwriter v1.2.0
+	github.com/oschwald/maxminddb-golang/v2 v2.7.0
 	github.com/prometheus/client_golang v1.24.1
 )
 
@@ -16,6 +18,7 @@ require (
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.70.1 // indirect
 	github.com/prometheus/procfs v0.21.1 // indirect
-	golang.org/x/sys v0.47.0 // indirect
+	go4.org/netipx v0.0.0-20231129151722-fdeea329fbba // indirect
+	golang.org/x/sys v0.48.0 // indirect
 	google.golang.org/protobuf v1.36.11 // indirect
 )