Fix the second review of the origin changes

Stop now counts the origin changes inside a running write, in a new
writingOriginChanges field, and adds them to the count it logs. The prune test
checks which rows survive, including a prefix with rows on both sides of 72
hours. The prefix page tests cover 5,000 and 5,001 rows and a row's time and
peer count. New tests cover the dropped-rows warning and the dropped count's
reset. The prune floor is named minOriginChangesPerPrefix. The README, TODO,
package doc and doc comments are corrected to match.

Model: deepseek-v4-pro

internal/database/originchanges.go

@@ -9,13 +9,13 @@ import (
 	"git.eeqj.de/sneak/routewatch/internal/routetable"
 )
 
-// maxOriginChangesPerPrefix is how many origin changes the prune keeps for
-// each prefix: the newest ones. A prefix with more has its older rows deleted.
-const maxOriginChangesPerPrefix = 500
+// minOriginChangesPerPrefix is how many of a prefix's newest origin changes
+// the prune always keeps, whatever their age.
+const minOriginChangesPerPrefix = 500
 
 // originChangesRetention is how long the prune keeps an origin change: rows
 // older than this are deleted, unless they are among the prefix's newest
-// maxOriginChangesPerPrefix.
+// minOriginChangesPerPrefix.
 const originChangesRetention = 72 * time.Hour
 
 // The statements of the origin_changes table (schema.sql).
@@ -122,13 +122,13 @@ func (d *Database) GetOriginChanges(
 }
 
 // PruneOriginChanges deletes, for each prefix with more than
-// maxOriginChangesPerPrefix rows, the rows older than originChangesRetention
-// that are not among its newest maxOriginChangesPerPrefix. It finds those
+// minOriginChangesPerPrefix rows, the rows older than originChangesRetention
+// that are not among its newest minOriginChangesPerPrefix. It finds those
 // prefixes on a reading connection and deletes one prefix per write, so no
 // write holds the lock for long.
 func (d *Database) PruneOriginChanges(ctx context.Context) error {
 	rows, err := d.db.QueryContext(ctx, prunePrefixesSQL,
-		maxOriginChangesPerPrefix)
+		minOriginChangesPerPrefix)
 	if err != nil {
 		return fmt.Errorf("failed to find prefixes to prune: %w", err)
 	}
@@ -158,7 +158,7 @@ func (d *Database) PruneOriginChanges(ctx context.Context) error {
 		d.lock("PruneOriginChanges")
 
 		_, err := d.writer.ExecContext(ctx, pruneOriginChangesSQL,
-			prefix, cutoff, prefix, maxOriginChangesPerPrefix-1)
+			prefix, cutoff, prefix, minOriginChangesPerPrefix-1)
 
 		d.unlock()