Add smtp_max_line_length_kb to accept non-conformant SMTP lines (fixes silent drops from e.g. Swann NVW-MR4K)

[zutroy97]

Problem

Some SMTP senders violate RFC 2045 §6.8 by encoding a base64 attachment as a single unbroken line with no CRLF wrapping. aiosmtpd (which this add-on is built on) enforces the RFC 5321 §4.5.3.1.6 line-length limit (~1000 octets) and aborts the DATA phase with 500 Line too long the moment it hits such a line — so the add-on never receives, and therefore never publishes, any message from that sender that includes such an attachment.

I hit this with a Swann NVW-MR4K (MaxRanger4K) security hub: its motion-alert email (JPEG snapshot attached) always triggers this, 100% of the time, while its plain verification/test email (no attachment, no long line) works fine — which made it look at first like an intermittent bug rather than the hub's SMTP client having a firmware quirk with no user-facing fix.

Fix

Adds an optional smtp_max_line_length_kb config option. When set, it raises aiosmtpd's SMTP.line_length_limit for the whole listener via a small Controller/SMTP subclass built in make_relaxed_line_length_controller(). Left unset (the default), the code path is unchanged — same Controller class, same RFC-compliant 1000-octet limit as today.

Verified against:

• A crafted message reproducing the exact malformation (a ~2.8MB single unbroken base64 line) — confirms 250 Message accepted for delivery instead of 500 Line too long, correct MIME parsing, and successful MQTT publish.
• The real Swann hub in production — motion alerts with photos now arrive and publish correctly, where every single one silently failed before.

Second commit: fix local builds under current Supervisor

Unrelated to the line-length fix, but needed to actually build and test this: the existing Dockerfile's ARG BUILD_FROM / FROM $BUILD_FROM relies on Supervisor auto-supplying BUILD_FROM from a build.yaml — a file this repo doesn't have. Supervisor 2026.04.0+ dropped the implicit fallback that used to paper over this, so a local build of the unmodified Dockerfile now fails outright:

ERROR: failed to build: failed to solve: base name ($BUILD_FROM) should not be blank

This commit hardcodes the base image directly (FROM ghcr.io/home-assistant/base:3.24), per the current Home Assistant add-on dev docs, and adds --break-system-packages to the pip3 install, since that base image enforces PEP 668 and the bare pip3 install fails immediately after the FROM fix with an externally-managed-environment error. Anyone building this add-on locally today will hit both issues.

Compatibility

• smtp_max_line_length_kb is optional and additive — existing installs and configs are unaffected if it's left unset.
• Also bumped config.yaml version to 1.0.5 and updated DOCS.md/CHANGELOG.md accordingly.