phase1-3: apply review fixes and docs alignment

Implement phase 1 and 2 review actions (DMARC UI CSS fixes, bounded DMARC ingest stdin,
rate-limit eviction hardening, settings/user handler cleanup, shared atomic/supervisor helpers)
and phase 3 documentation/changelog updates including roadmap and accepted-risk notes.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/panel/dmarc_ingest.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"path/filepath"
@@ -12,6 +13,8 @@ import (
 	"github.com/mixeme/selfpost/internal/store"
 )
 
+const maxIngestStdin = 10 << 20 // 10 MiB
+
 func runDMARCIngestMode() error {
 	cfg := loadConfig()
 	if !cfg.dmarcEnabled {
@@ -27,7 +30,7 @@ func runDMARCIngestMode() error {
 	if recipient == "" {
 		recipient = strings.ToLower(strings.TrimSpace(os.Getenv("ORIGINAL_RECIPIENT")))
 	}
-	if err := dmarc.IngestMessage(st, os.Stdin, recipient, time.Now().UTC()); err != nil {
+	if err := dmarc.IngestMessage(st, io.LimitReader(os.Stdin, maxIngestStdin), recipient, time.Now().UTC()); err != nil {
 		if incrErr := st.IncrDMARCParseFailures(); incrErr != nil {
 			log.Printf("dmarc ingest: record failure: %v", incrErr)
 		}