feat: add GeoIP country lookup with embedded DB-IP Lite database (PLAN.md 4.3)

Embeds a DB-IP Lite country MMDB (~5MB) in the binary via go:embed,
keeping the single-binary deployment story clean. Country codes are
stored alongside login attempts and sessions, shown in the dashboard
(Top IPs, Top Countries card, Recent/Active Sessions, session detail).

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

PLAN.md

@@ -196,7 +196,7 @@ Goal: Make the web UI great and add operational niceties.
 - Docker image (nix dockerTools) ✅
 - Systemd unit file / deployment docs ✅
 
-### 4.3 GeoIP
-- Embed a lightweight GeoIP database or use an API
-- Store country/city with each attempt
-- Aggregate stats by country
+### 4.3 GeoIP ✅
+- Embed a lightweight GeoIP database or use an API ✅
+- Store country/city with each attempt ✅
+- Aggregate stats by country ✅