feat(email): include notification level in email subject line

[ch8matt]

Adds the NotificationRule's level (e.g. [WARNING], [ERROR]) to the subject of outgoing SMTP emails. Improves clarity and urgency of notifications received by users. Uses ctx.ruleLevel() for formatting.

Description

Adds the notification level (e.g., [WARNING], [ERROR], [INFORMATIONAL]) to the subject line of outgoing SMTP notification emails. This improves visibility and urgency of alerts directly from the subject without needing to open the email.

Addressed Issue

Additional Details

Checklist

• [x] I have read and understand the contributing guidelines
• [ ] This PR fixes a defect, and I have provided tests to verify that the fix is effective
• [x] This PR implements an enhancement, and I have provided tests to verify that it works as intended
• [ ] This PR introduces changes to the database model, and I have added corresponding update logic
• [ ] This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[owasp-dt-bot]

:white_check_mark: Snyk checks have passed. No issues have been found so far.

Status	Scanner	Critical	High	Medium	Low	Total (0)
:white_check_mark:	Open Source Security	0	0	0	0	0 issues

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[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.00% (target: -1.00%)	:white_check_mark: 100.00% (target: 70.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (fa1eb0bb4c1ecf87d231a21e077055acb6b8b59d)	24093	19492	80.90%
Head commit (c78b93b5632d2b301f68224b81c0b1ca6bd5d1ed)	24098 (+5)	19497 (+5)	80.91% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#5503)	6	6	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[lukas-braune]

"INFORMATIONAL" can be interpreted as "no action required" by organizations. Please consider allowing this feature to be disabled.

[ch8matt]

https://github.com/DependencyTrack/frontend/pull/1379

[ch8matt]

A new toggle, disabled by default, has been added to enable the feature.