Add statistics view, editable source_url, rewrite security audit

Statistics
- New `stats` module computes totals, top-N channels, per-year upload
  histogram, and per-week download activity from the in-memory library
  + watched/positions data.
- GET /api/stats endpoint returns the full report as JSON.
- Web UI: 📊 button in the header opens a stats modal with summary
  tiles, two CSS bar charts (weeks + years), and side-by-side top-N
  tables.
- Desktop GUI: 📊 Stats toggle in the top bar opens a window with
  the same data, rendered via egui rects (stdlib calendar math so
  there's no chrono dep).
- Bar chart helper `draw_bars` is reusable across stats sections.

Source URL (AGPL §13)
- `web.source_url` is now editable in both Settings UIs — was
  previously documented-only and required hand-editing config.toml.
- Settings POST persists it and the footer link updates immediately.
- Default config.toml ships with the Codeberg URL set.

SECURITY_AUDIT.md
- Re-audited the codebase as of commit 5999673; rewrote the file to
  document the threat model and which adversary each defense addresses.
- Resolved-from-2026-05-17 column shows what landed: session expiry,
  Secure cookie, rate-limit, body cap, CSP, X-Frame-Options,
  X-Content-Type-Options, safeUrl(), SHA-256 verify, chmod 0600 db.
- Documented accepted risks (plain-HTTP LAN, cookies.txt umask,
  job log stderr verbatim).
- Future-hardening checklist preserved for posterity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.rs

@@ -20,6 +20,7 @@ mod downloader;
 mod library;
 mod maintenance;
 mod plex;
+mod stats;
 mod theme;
 mod web;
 mod ytdlp_bin;