import: the export file held to the forms' rules before anything is created (audit M10)

domain.Import trusted the file for everything but the domain name, the
selector and the password: DMARCRua was not validate.Email'ed, client IPs
not parsed, a rate limit's mode, window and count not checked - a window of
0 counted nothing, a garbage IP made the application refuse every client.

handlers.checkDomainImport runs in HandleImportDomain after the domain name
check and before Import, and refuses the file with "Invalid <what> in export
file: <the form's own text>": the report address (validate.Email), each
rate limit (parseRateLimitMode; auto - checkAutoMultiplier, 0 meaning the
default as an empty field does; manual - checkManualLimit, which the form
now uses too: count > 0, <= the level-1 backstop, window > 0; both 0 is an
export without a limit of its own), each application's client addresses
and the legacy ones on its rate limit (parseIPList, written back as the
form stores them; allow-list on with none is refused). Login, sending
addresses and password stay checked in app.ImportApplication.

Tests: a good export passes and its addresses come out normalised; nine
broken fields each refused with their message; a refused file posted to the
handler creates no domain.

CHANGELOG [Unreleased], audit plan line ticked, roadmap 18/62.
Evidence: go vet clean; the import, rate-limit and application tests of
./internal/web/handlers ok; the server run follows before the push.

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

docs/roadmap.md

@@ -42,7 +42,7 @@ in `git log` and [CHANGELOG.md](../CHANGELOG.md).
 | from-header-binding | Hold a message's `From:` header against the application that sends it | shipped `2.4.0` | 9/9 | [plans/from-header-binding.md](plans/from-header-binding.md) |
 | panel-redesign | Panel redesign (accepted mockups, design contract) | shipped `2.0.0` | 27/27 | [plans/panel-redesign.md](plans/panel-redesign.md) |
 | schema-squash | Squash SQLite migrations into a 2.x baseline | built in panel-redesign (stage 1) | — | [plans/panel-redesign.md](plans/panel-redesign.md) |
-| audit-2026-10-12 | Follow-ups of the 2026-10-12 full-tree code audit, seven stages | in progress (stage 0 shipped `2.4.2`; next stages 1+2 → `2.5.0`) | 17/62 | [audit-2026-10-12.md](audit-2026-10-12.md) |
+| audit-2026-10-12 | Follow-ups of the 2026-10-12 full-tree code audit, seven stages | in progress (stage 0 shipped `2.4.2`; next stages 1+2 → `2.5.0`) | 18/62 | [audit-2026-10-12.md](audit-2026-10-12.md) |
 
 **Recommended order** (not binding; owner, 2026-10-09: feature order follows
 development convenience): **panel-redesign** shipped as `2.0.0` (2026-10-11) and