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>

internal/web/handlers/handlers_dmarc_pages_test.go

@@ -49,7 +49,7 @@ func TestDMARCHubShowsIngestAndRecentReports(t *testing.T) {
 	body := getBody(t, s.h.HandleDMARCList, "/outbound/dmarc")
 	has(t, "hub", body,
 		`<h1 class="title is-3">DMARC reports</h1>`, `<h2>Ingest</h2>`, `<span class="tag is-success is-light">ok</span>`,
-		`1 kept · 0 parse failures`, `Hosted addresses`, `dmarc-reports&#43;&lt;domain&gt;@mail.example.org`, `500 reports or 90 days`,
+		`1 kept · 0 parse failures`, `Hosted addresses`, `dmarc-reports&#43;&lt;domain&gt;@mail.example.org`, `500 reports a domain or 90 days`,
 		`<h2>Recent reports</h2>`, `<a href="/outbound/dmarc/domains/1">example.org</a>`, `Outlook.com`,
 		"<td>1\u00a0903</td>", `<span class="tag is-warning is-light">7</span>`, `href="/outbound/dmarc/reports/`+idStr(s.reportID)+`">View</a>`)
 	lacks(t, "hub", body, `None yet`)