dmarc: a report is bounded, kept per domain, and ingested one at a time

Security review of the DMARC report path (Fable, 2026-10-11), which ran
for the first time in 2.0.0. No Critical or High finding. Fixed:

- Medium: a report was bounded only as 8 MiB of XML - some 160 000
  records or one name of megabytes, all of it rows and table cells. At
  most 5 000 records (more is ErrRejected, dropped), text fields cut to
  255 bytes on a character boundary, a record's count capped.
- Medium: the cap of 500 reports was for the whole server, so a stranger
  filing under one domain's address evicted every other domain's
  reports. PruneDMARCReports counts per domain.
- Low: the pipe service ran up to default_process_limit (100) commands
  at once against the panel's database; maxproc is 1.
- Low: the reason logged for a dropped message carried the sender's
  bytes (header line, zip entry name) as they came; cut to 200 bytes and
  reduced to printable ASCII, zip entry names quoted.
- Low: SELFPOST_DATA_DIR / SELFPOST_DB_PATH with whitespace or "$" would
  reach the command through export_environment as another path; refused
  at start when DMARC ingest is on.

Left as they are, told to the owner: the parse-failure count is a
lifetime figure shown under "This week"; a report larger than the
command reads is dropped as unreadable; nested multipart is not searched.

The hub's Retention line follows the design commit before this one.
security.md: the review, and the accepted-risk entry says what a
stranger can and cannot do now. The roadmap no longer calls the review
pending.

  go vet ./cmd/... ./internal/dmarc/ ./internal/store/: ok
  go test ./cmd/... ./internal/dmarc/ ./internal/store/ ./internal/web/...: ok
  make e2e on the test server (with the port-25 scenarios of the next
  commit): ok

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build/postfix-config.sh

@@ -291,11 +291,23 @@ if [ "${INBOUND_RELAY_ENABLE}" = "true" ] || [ "${DMARC_REPORTS_ENABLE}" = "true
 		# attribute" on every report. They go into the service line itself.
 		# ${recipient} is expanded by pipe(8), not by the shell: the command
 		# learns which report address the message came to from its argument.
-		postconf -M 'dmarc-ingest/unix=dmarc-ingest unix - n n - - pipe flags=Rq user=panel argv=/usr/local/bin/dmarc-ingest ${recipient}'
+		# One at a time (the 1 before "pipe"): reports are in no hurry, every run
+		# writes to the database the panel writes to, and a hundred of them in
+		# parallel - Postfix's default - is what a flood of reports would start.
+		postconf -M 'dmarc-ingest/unix=dmarc-ingest unix - n n - 1 pipe flags=Rq user=panel argv=/usr/local/bin/dmarc-ingest ${recipient}'
 		# One recipient per run, so ${recipient} is one argument.
 		postconf -e "dmarc-ingest_destination_recipient_limit=1"
 		# pipe(8) starts the command with export_environment only, so what the
 		# command reads from the container's environment is named there.
+		# export_environment is a list split at whitespace in which Postfix
+		# expands "$": a path with either would reach the command as another
+		# path, and it would quietly open a database of its own.
+		case "${SELFPOST_DATA_DIR:-}${SELFPOST_DB_PATH:-}" in
+			*[[:space:]]* | *'$'*)
+				echo "postfix-config: SELFPOST_DATA_DIR and SELFPOST_DB_PATH must not contain whitespace or \$ when DMARC_REPORTS_ENABLE=true" >&2
+				exit 1
+				;;
+		esac
 		DMARC_INGEST_ENV="DMARC_REPORTS_ENABLE=true"
 		[ -z "${SELFPOST_DATA_DIR:-}" ] || DMARC_INGEST_ENV="${DMARC_INGEST_ENV} SELFPOST_DATA_DIR=${SELFPOST_DATA_DIR}"
 		[ -z "${SELFPOST_DB_PATH:-}" ] || DMARC_INGEST_ENV="${DMARC_INGEST_ENV} SELFPOST_DB_PATH=${SELFPOST_DB_PATH}"