feat(triage signals): Add billing flag to all triage signals check

[jasonyuezhang]

PR Details

• Replace direct features.has("organizations:triage-signals-v0-org") checks with is_seer_seat_based_tier_enabled() across all triage signals code paths.
• Already tested it out on s amller scale with this PR that just added it in 1 place: feat(triage signals): Use new seat based pricing feature flag getsentry/sentry#104528. Logs are here. No failure logs yet.

Changes

• issue_summary.py: Updated run_automation() to use is_seer_seat_based_tier_enabled() for both the ALERT event count check and the stopping point logic
• similarity/utils.py: Updated set_default_project_autofix_automation_tuning() and set_default_project_seer_scanner_automation() to use is_seer_seat_based_tier_enabled()
• post_process.py: Updated kick_off_seer_automation() to use is_seer_seat_based_tier_enabled()

Why

The is_seer_seat_based_tier_enabled() function checks both:

1. The existing organizations:triage-signals-v0-org feature flag
2. The new organizations:seat-based-seer-enabled billing flag (with caching)

This allows triage signals features to work for organizations with either the feature flag OR the seat-based billing tier enabled.

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Copied from getsentry#104570
Original PR: getsentry#104570