feat: grant Elysian role on auth and prompt non-members to join (#134)

## Summary

- Grants the Elysian Discord role to players on login/registration and persists an `inGuild` flag on the Player record
- Connects to the Discord Gateway via WebSocket to keep `inGuild` in sync as players join or leave the server
- Shows a dismissible "Join our community" modal to players who are not yet in the guild
- Hardens `inGuild` exposure through the load endpoint and game context
- Moves all non-secret Discord IDs (guild, role, client, redirect URI) out of env vars and into hardcoded constants; removes them from `prod.env`

## Test plan

- [ ] Lint, build, and test pipeline passes (100% coverage maintained)
- [ ] New player auth grants Elysian role and sets `inGuild: true`
- [ ] Existing player auth re-attempts role grant and updates `inGuild`
- [ ] Join community modal appears for players not in the guild
- [ ] Modal does not reappear within the same browser session after dismissal
- [ ] Gateway correctly sets `inGuild: true/false` on member add/remove events

✨ This issue was created with help from Hikari~ 🌸

Reviewed-on: #134
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/api/src/services/discord.ts

@@ -7,6 +7,9 @@
 /* eslint-disable @typescript-eslint/naming-convention -- Discord API requires snake_case fields and HTTP headers require Pascal-Case */
 import { logger } from "./logger.js";
 
+const discordClientId = "1479551654264049908";
+const discordRedirectUri = "https://elysium.nhcarrigan.com/api/auth/callback";
+
 interface DiscordTokenResponse {
   access_token:  string;
   token_type:    string;
@@ -31,24 +34,18 @@ interface DiscordUser {
 const exchangeCode = async(
   code: string,
 ): Promise<DiscordTokenResponse> => {
-  const clientId = process.env.DISCORD_CLIENT_ID;
   const clientSecret = process.env.DISCORD_CLIENT_SECRET;
-  const redirectUri = process.env.DISCORD_REDIRECT_URI;
 
-  if (
-    clientId === undefined || clientId === ""
-    || clientSecret === undefined || clientSecret === ""
-    || redirectUri === undefined || redirectUri === ""
-  ) {
+  if (clientSecret === undefined || clientSecret === "") {
     throw new Error("Discord OAuth environment variables are required");
   }
 
   const parameters = new URLSearchParams({
-    client_id:     clientId,
+    client_id:     discordClientId,
     client_secret: clientSecret,
     code:          code,
     grant_type:    "authorization_code",
-    redirect_uri:  redirectUri,
+    redirect_uri:  discordRedirectUri,
   });
 
   try {
@@ -146,19 +143,9 @@ const fetchDiscordUserById = async(
  * @throws {Error} If OAuth environment variables are missing.
  */
 const buildOAuthUrl = (): string => {
-  const clientId = process.env.DISCORD_CLIENT_ID;
-  const redirectUri = process.env.DISCORD_REDIRECT_URI;
-
-  if (
-    clientId === undefined || clientId === ""
-    || redirectUri === undefined || redirectUri === ""
-  ) {
-    throw new Error("Discord OAuth environment variables are required");
-  }
-
   const parameters = new URLSearchParams({
-    client_id:     clientId,
-    redirect_uri:  redirectUri,
+    client_id:     discordClientId,
+    redirect_uri:  discordRedirectUri,
     response_type: "code",
     scope:         "identify",
   });