fix: resolve security, reliability, and performance issues found in code review

[ErikKalkoken]

This PR fixes 19 security, reliability, and performance issues found in code review by Claude AI. All issues and fixes have been reviewed by me.

• Security fix: closed a gap where a user could view another corporation's or alliance's structure fittings, POCO details, or starbase details by guessing the URL, even without permission to see that structure.
• Notifications no longer get lost: Discord notifications that failed to send due to a network hiccup are now properly retried instead of silently disappearing.
• Notifications no longer get duplicated: fixed a timing issue that could cause the same alert to be posted to Discord twice.
• Structure-specific webhooks now work correctly: structures configured with more than one dedicated webhook now actually send to all of them, instead of quietly falling back to the corporation's default channels.
• Fixed a crash when generating a "jump fuel running low" alert for a structure that had already been removed.
• The admin "purge queued messages" action and queue-size display now correctly account for messages waiting to be retried, not just newly queued ones.
• Corporation sync reports now show the real number of notifications sent, instead of always showing zero.
• Moon extraction "cancelled" notifications are no longer mistakenly marked as handled when no matching timer could be found, so stale timers can still be cleaned up.
• The admin "process for timerboard" action now correctly tells you how many notifications were skipped, not just how many were processed.
• Fixed several notification types (corp applications, corp goals, skyhook attacks) that weren't being translated correctly for non-English users.
• Fixed crashes that could occur from an invalid structure list filter or an invalid character selection on the public page — these now show a normal error instead of a server crash.
• Faster loading of the structures list and reinforcement-time-changed notifications, especially for larger corporations and alliances, thanks to reduced database load.

[codecov]

Codecov Report

❌ Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
structures/core/serializers.py	50.00%	0 Missing and 2 partials ⚠️

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