[P2.1] Costs `/tasks` endpoint N+1 (connection-per-task + per-task subquery)

[frankbria]

Severity: high | Category: infra | Phase: P2.1

Problem

_query_top_tasks calls tasks.get() per row (new SQLite connection each) with limit up to 1000, and get_top_tasks_by_cost runs a dominant-agent subquery per task — ~1000 connection opens + ~2000 queries for one dashboard request.

Evidence

codeframe/ui/routers/costs_v2.py:232, codeframe/platform_store/repositories/token_repository.py:419

Acceptance criteria

Title lookup batched via WHERE id IN (...); dominant-agent folded into one windowed/CTE query; O(1) queries regardless of limit.

Dependencies

blocked by #712 ([P0.1])

---

_{Filed from the SaaS launch-readiness audit. Atomic: one developer, one session. Work order: strictly P0.1 → P3.12 (no forward dependencies).}

[coderabbitai]

🔗 Related PRs

#131 - Implement database-backed tasks endpoint with security fixes [merged]
#152 - Implement agent maturity level system [merged]
#596 - fix(server): slim control-plane schema; unblock codeframe serve [merged]
#688 - fix(core): release tasks.py DB connections on exception + retain fire-and-forget auto-close task (#650) [closed]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!