feat: implement C.4 — hermetic container e2e gate + native release build

Separate test/e2e Go module drives the shipped deploy/docker-compose.yml
(plus a test-only override: self-signed cert, low ports, isolated compose
project) against a fake DNS zone (CoreDNS) and an smtp-sink MX, exactly as
an administrator and their applications would over HTTP/SMTP — covering the
class of failure unit tests can't see (container wiring). Positive path:
setup -> login -> domain -> DKIM record published into the fake zone ->
application -> SMTP AUTH send -> DKIM verified against the DNS-published
key -> send-log queued->sent. Negative: no-AUTH/unauthenticated relay,
sender/login mismatch, L1 (anvil) and L2 (panel) rate limits, journal-milter
fail-open, SELFPOST_HOSTNAME gate, session survives docker restart.

release.yml moves off qemu to a native per-arch build (amd64/arm64), each
gated by this suite before its tag is pushed and merged into the version
manifest.

Verified green on selfpost.example.com via `make e2e`; go vet/gofmt clean in
both modules.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/e2e/go.sum

@@ -0,0 +1,4 @@
+github.com/emersion/go-msgauth v0.6.8 h1:kW/0E9E8Zx5CdKsERC/WnAvnXvX7q9wTHia1OA4944A=
+github.com/emersion/go-msgauth v0.6.8/go.mod h1:YDwuyTCUHu9xxmAeVj0eW4INnwB6NNZoPdLerpSxRrc=
+golang.org/x/crypto v0.15.0 h1:frVn1TEaCEaZcn3Tmd7Y2b5KKPaZ+I32Q2OA3kYp5TA=
+golang.org/x/crypto v0.15.0/go.mod h1:4ChreQoLWfG3xLDer1WdlH5NdlQ3+mwnQq1YTKY+72g=