feat: add sync statistics to dashboard

[furch-services]

What problem does this solve?

The dashboard shows only the outcome of the most recent sync. There is no visibility into trends — whether errors are increasing, how many documents are being synced per week, or whether the scheduler is running reliably over time.

Proposed solution

Add a statistics section to the dashboard with a 14-day activity chart and aggregate counters. Data is served from a new /api/stats endpoint and rendered with Chart.js via CDN.

• Bar chart: daily upload count for the last 14 days
• Stat cards: total uploaded (all-time), total skipped, total errors, permanently failed
• /api/stats returns JSON, derived from existing sync_log data — no new tables
• Chart updates without full reload via HTMX (depends on feat/htmx-ui)

Acceptance criteria:

• /api/stats endpoint returns daily counts for the last 14 days
• Chart.js bar chart rendered from that data (CDN, no build step)
• Aggregate stat cards displayed on the dashboard
• Chart is responsive on narrow viewports
• No additional database tables required

Alternatives considered

Server-side SVG chart generation (without Chart.js) was considered to avoid any JS dependency, but Chart.js via CDN is a single script tag with no build step and produces much better output.

Architecture fit

• Does not require Redis, RabbitMQ, Celery, PostgreSQL, React, Vue, or Node.js
• Runs inside the single application container (no separate worker or sidecar)