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>

internal/storage/migrations/003_add_country.sql

@@ -0,0 +1,3 @@
+ALTER TABLE login_attempts ADD COLUMN country TEXT NOT NULL DEFAULT '';
+ALTER TABLE sessions ADD COLUMN country TEXT NOT NULL DEFAULT '';
+CREATE INDEX idx_login_attempts_country ON login_attempts(country);