feat: dashboard trend sparklines + history time-series

[Jamkris]

Summary

Records dashboard counters over time and shows lightweight trend sparklines on the home page.

• New bastion/history.py: append-only JSONL time-series under the data dir, capped at ~24h (auto-trim). Best-effort IO, never raises.
• The background poller records a snapshot each cycle (independent of alerts).
• Pure sparkline_points() renders an inline SVG polyline server-side — no charting library / CDN.
• Home shows banned + attacker sparklines (last ~hour). New GET /api/history?limit=N.

Test plan

• python -m pytest -q → 125 passed. New: tests/test_history.py, sparkline cases in tests/test_util.py, poller/web integration tests.
• data/history.jsonl is gitignored.

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Summary by cubic

Adds trend sparklines to the home page for banned IPs and attackers. A new capped JSONL history records counters and powers a simple history API.

• New Features
  • Append-only JSONL time-series of dashboard counters, capped at ~24h; best-effort IO.
  • Server-rendered SVG sparklines (no charting library) on the home page using recent samples (~last hour).
  • New GET /api/history?limit=N returning recorded points for external use.

^{Written for commit b230a2f. Summary will update on new commits.}