Send Gateway reconnect email

[moubctez]

#2359

[Copilot]

Pull request overview

This PR extends gateway email notification behavior by adding a “gateway reconnected” notification (with an inactivity threshold), and adjusts build-time version metadata generation in defguard_common.

Changes:

• Add reconnect email notification flow in GatewayHandler and trigger it on successful gateway connection timestamp update.
• Update mail handler API for reconnect notifications to require a non-optional gateway name.
• Simplify defguard_common build metadata generation by removing branch emission and adjusting vergen-git2 dependency usage.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/defguard_gateway_manager/src/handler.rs	Adds reconnect notification logic and refines disconnect notification gating; triggers reconnect email after touch_connected.
crates/defguard_core/src/handlers/mail.rs	Changes send_gateway_reconnected_email to take gateway_name: String (non-optional).
crates/defguard_common/Cargo.toml	Simplifies vergen-git2 build-dependency declaration.
crates/defguard_common/build.rs	Stops emitting git branch metadata; emits SHA only.
Cargo.lock	Updates lockfile to reflect dependency graph changes.

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