feat(admin): add monthly gateway model usage report

[chrarnoldus]

Summary

• Add a Monthly Usage tab to Gateway admin with a UTC year/month picker and requested-model input, including current-model suggestions and historical IDs.
• Query the PostgreSQL read replica through readDb, joining microdollar_usage to microdollar_usage_metadata by ID and grouping by provider and is_byok.
• Filter requested_model and is_user_byok = false exactly; use half-open UTC created_at bounds and case-insensitive anonymous-user exclusion for logged-in counts.
• Include input/output/cache-read/cache-write totals, cost, and market cost. Cast aggregate values to text to preserve precision; costs are explicitly labeled microdollars.
• Display and copy header-bearing TSV for spreadsheets, with a selectable-text fallback and formula-safe provider fields.
• Set a transaction-local statement_timeout of at least ten minutes, respecting larger configured admin timeouts. Refuse production fallback to the primary database.
• Removed all Snowflake-specific changes from this PR.

Validation

• Formatting and git diff --check passed; changed UI lint passed.
• Updated backend lint/typecheck were attempted but exceeded this container's two-minute limit. Local Node is 22; repository requires 24.
• Added replica transaction, timeout, input/auth, precision, and real PostgreSQL aggregation regression coverage. Tests run in CI as requested.
• UI retains the Kilo Cloud overlay and tabs recipe.

Runtime note

• Existing tRPC maxDuration is 800 seconds; larger configured statement timeouts remain subject to this hosting limit.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (7 files)

• apps/web/src/app/admin/gateway/MonthlyUsageContent.tsx
• apps/web/src/app/admin/gateway/monthly-usage-export.test.ts
• apps/web/src/app/admin/gateway/monthly-usage-export.ts
• apps/web/src/app/admin/gateway/page.tsx
• apps/web/src/routers/admin-router.ts
• apps/web/src/routers/admin/gateway-usage-router.test.ts
• apps/web/src/routers/admin/gateway-usage-router.ts

Previous Review Summary (commit ad69910)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit ad69910)

Status: No Issues Found | Recommendation: Merge

Files Reviewed (9 files)

• apps/web/src/app/admin/gateway/MonthlyUsageContent.tsx
• apps/web/src/app/admin/gateway/monthly-usage-export.test.ts
• apps/web/src/app/admin/gateway/monthly-usage-export.ts
• apps/web/src/app/admin/gateway/page.tsx
• apps/web/src/lib/snowflake.test.ts
• apps/web/src/lib/snowflake.ts
• apps/web/src/routers/admin-router.ts
• apps/web/src/routers/admin/gateway-usage-router.test.ts
• apps/web/src/routers/admin/gateway-usage-router.ts

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_{Review guidance: REVIEW.md from base branch main}