feat: add client aggregate APIs

[zhangyangyu]

Description

Add reusable aggregate endpoints for viewer bootstrap, repo summaries, org management summaries, issue threads, wiki catalogs/batch pages, and notification summaries.

Changes:

• extend issue listing with server-side title prefix filtering and lightweight field selection
• document the aggregate API design
• add an explicit E2E benchmark script under e2e/benchmark/ so default make test-e2e does not run it

Tracking

N/A

Tests

• go test ./internal/rest -run 'Test.*Aggregate|Test.*Summary|Test.*IssueThread|Test.*Wiki'
• go test ./internal/service
• bash -n e2e/benchmark/client-aggregate-performance.sh
• SCRIPT=benchmark/client-aggregate-performance bash e2e/run.sh skips without token as expected
• go test ./... -run '^$'
• python3 scripts/doc_lint.py
• git diff --check

Notes

• make test-e2e keeps its existing top-level script discovery behavior. Benchmark scripts live under e2e/benchmark/ and must be run explicitly with SCRIPT=benchmark/client-aggregate-performance.
• Focused full runtime tests can require a running TiDB test database; this PR was compile-checked across all packages with go test ./... -run '^$'.

SSOT Drift Prevention Checklist

• Checked that this PR does not violate boundaries in docs/architecture.md or docs/module-contracts.md
• If this PR changes package structure or dependencies, updated the relevant SSOT doc
• If this PR changes test conventions or coverage expectations, updated docs/test-strategy.md
• If this PR changes CI, smoke, regression, or workflow expectations, updated docs/ci.md