feat(weekly-reports): Add data layer for Total Issues graph

[amy-chen23]

Addresses data layer of ID-1669

Goal: Add Total Issues graph alongside existing Total Errors graph. We want to remove "Total Project Transactions" entirely --> we'll do this in a separate PR though

Changes

• Add issue_count_by_day and prev_week_total_substatus_count to ProjectContext for per-project daily issue counts and WoW comparison
• Add organization_project_issue_counts_by_day(start, end, ctx) query using TruncDay("last_seen") on unresolved groups --> TruncDay follows UTC, so it still matches Snuba time windows for total error counts too
• Cache issue counts ("i" key) alongside errors/transactions for WoW
• Rename _append_project_event_counts_previous_week → _append_previous_week_counts to reflect broader scope
• Extend trends() to include issue_count in series and substatus counts in legend for this chart

TODO:

• Rendering PR (next) will use this data to display the chart and updated table

[amy-chen23]

@sentry review

[amy-chen23]

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