Fix the origin-changes review findings

Hold the 100,000-row cap after a failed write's rows are put back, dropping and counting the oldest rows past it. Stop counts the unwritten origin changes. The prefix handler's per-write origin change limit is a field, so a test checks that one write leaves the rest. The server and README now say where origin changes come from. README, TODO and schema say 72 hours or 500 rows, whichever keeps more. The prefix page and TODO list the origin change columns and say what the peer count counts. The page takes its 5,000-row note from the constant.

Model: deepseek-v4-pro

internal/server/handlers.go

@@ -1637,7 +1637,9 @@ type prefixDetailData struct {
 	OriginCount       int
 	OriginChanges     []routetable.OriginChange
 	MoreOriginChanges bool
-	CSPNonce          string
+	// MaxOriginChanges is the most origin changes the page shows.
+	MaxOriginChanges int
+	CSPNonce         string
 }
 
 // newPrefixDetailData builds the prefix detail page's data.
@@ -1655,6 +1657,7 @@ func newPrefixDetailData(
 		OriginCount:       len(originInfos),
 		OriginChanges:     changes,
 		MoreOriginChanges: moreChanges,
+		MaxOriginChanges:  maxOriginChangesOnPage,
 		CSPNonce:          nonce,
 	}
 }

internal/server/prefix_originchanges_internal_test.go

@@ -142,7 +142,7 @@ func TestPrefixDetailSaysWhenMoreThan5000Changes(t *testing.T) {
 
 	page := rec.Body.String()
 
-	if !strings.Contains(page, "more than 5,000 changes") {
-		t.Errorf("page does not say there are more than 5,000 changes")
+	if !strings.Contains(page, "more than 5000 changes") {
+		t.Errorf("page does not say there are more than 5000 changes")
 	}
 }

internal/server/server.go

@@ -7,8 +7,8 @@
 //   - the AS name search
 //
 // Every route it shows comes from the routing table in memory. AS names,
-// WHOIS details and the times a prefix and an AS were first seen come from
-// the database.
+// WHOIS details, the times a prefix and an AS were first seen, and a prefix's
+// origin changes come from the database.
 package server
 
 import (