Add web admin panel for accounts/characters (login, GM rights, live stats)

New "webadmin" service (Flask + PyMySQL, port 8081) backing a small
server-rendered admin UI:
- Login gated on an existing game account: only accounts with
  accesslevel >= ADMIN_MIN_ACCESSLEVEL (default 900) get in. Password
  is MD5-hashed before comparison since the game client hashes it
  client-side before it ever hits the wire -- accounts.password is
  always an MD5 hex digest, confirmed against existing DB rows.
- Accounts: list/search, create, edit accesslevel/active/password.
- Characters: list/search, create (mirrors the real char-creation
  INSERT in Src/Char Server/charpackets.cpp -- only account_name,
  char_name, face, hairStyle, sex are set, everything else relies on
  the table's own defaults), edit core stats (level, zuly, attributes,
  hp/mp, stat/skill points) and isgm/isdev flags directly.

All queries are parameterized (no string-formatted SQL). No CSRF
protection or rate limiting yet -- acceptable for a small
already-behind-a-high-accesslevel-gate private server, worth
revisiting before ever exposing this port publicly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -34,3 +34,10 @@ LOGIN_CREATE_ACCOUNT=1
 SHARED_LOGIN_PASS=789456
 SHARED_CHAR_PASS=456123
 SHARED_WORLD_PASS=321654
+
+# --- Web admin panel (http://<host>:8081) -----------------------------------
+# Logs in with an existing game account; only accounts with
+# accesslevel >= ADMIN_MIN_ACCESSLEVEL are allowed in.
+# Generate your own with: python3 -c "import secrets; print(secrets.token_hex(32))"
+ADMIN_SECRET_KEY=change-me-to-a-random-value
+ADMIN_MIN_ACCESSLEVEL=900