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

internal/proxy/request.go

@@ -15,6 +15,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"sneak.berlin/go/smallwebwaf/internal/lookup"
 	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
@@ -49,9 +50,11 @@ type request struct {
 	peer        netip.Addr
 	peerTrusted bool
 	// lookedUp is true once the client's AS number and country have been
-	// looked up, whether or not an answer was there.
-	lookedUp bool
-	start    time.Time
+	// looked up, whether or not an answer was there, and lookupAnswer is
+	// what the lookup gave then, the zero Answer while GeoJS had given none.
+	lookedUp     bool
+	lookupAnswer lookup.Answer
+	start        time.Time
 	// checked is when the checks were done, and upstreamStart when the
 	// request was handed to the app.
 	checked       time.Time
@@ -488,10 +491,11 @@ func timing(start, end time.Time) *float64 {
 }
 
 // addToHistory adds the request, which has ended, to its client's
-// history, and then, for a client that was looked up, the answer kept
-// about it to that history and to the notes of the bans on its netblock:
-// an answer that came during the request may have come before either was
-// there, and one that comes later is added when it comes.
+// history, and then, for a client that was looked up, the lookup
+// database's answer about it, or the answer from GeoJS kept about it, to
+// that history and to the notes of the bans on its netblock: an answer
+// may have come before either was there, and one from GeoJS that comes
+// later is added when it comes.
 func (rq *request) addToHistory() {
 	var requestBytes int64
 	if rq.body != nil {
@@ -514,6 +518,13 @@ func (rq *request) addToHistory() {
 		return
 	}
 
+	// The lookup database's answer was there at once.
+	if rq.h.config.LookupSource == "file" {
+		rq.h.addLookup(rq.lookupAnswer)
+
+		return
+	}
+
 	answer, kept := rq.h.geojs.Kept(group)
 	if kept {
 		rq.h.addLookup(answer)