perf: compact repeated request history prefixes

[liaohch3]

Summary

• Add SQLite schema v5 prefix-delta compaction for repeated request.body.input and request.body.messages histories.
• Keep existing v3/v4 SQLite data readable; v4 -> v5 only adds the new schema version marker and does not rewrite existing rows.
• Add anchor-aware pagination, boundary reads, JSONL/compact export materialization, and an append-time last-record cache so large sessions stay fast.

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run pytest tests/ -x --timeout=60 -> 625 passed, 25 skipped
• uv run pytest tests/test_trace_store_compaction.py tests/test_history.py -q -> 24 passed
• Real-shape storage validation: seeded from .traces/codex-tool-mechanisms-20260509-more/2026-05-09/trace_083536.jsonl, wrote 60 Codex-style growing-history records into SQLite v5, verified random pagination, boundary reads, JSONL export, compact export, and materialization. Result: 57 prefix-delta rows, 16,359,129 raw JSONL bytes vs 916,272 compact bytes, storage ratio 0.056.
• Legacy SQLite validation: copied .traces/pr269-bedrock-evidence/traces.sqlite3 and .traces/pr269-newapi-bedrock-claude/traces.sqlite3, loaded/exported sessions successfully, and confirmed schema v5 migration.
• Evidence screenshot validation: uv run python scripts/check_screenshots.py .agents/evidence/pr/pr-sqlite-prefix-compaction-viewer.png

Evidence

• Real trace source: .traces/codex-tool-mechanisms-20260509-more/2026-05-09/trace_083536.jsonl
• Viewer screenshot from SQLite v5 materialized export: https://raw.githubusercontent.com/liaohch3/claude-tap/feat/sqlite-prefix-compaction/.agents/evidence/pr/pr-sqlite-prefix-compaction-viewer.png

Privacy

• This PR does not commit raw trace JSONL, generated HTML viewers, logs, API keys, or auth tokens.