fix: 3 HIGH + 4 MEDIUM + 4 LOW bugs, env vars, security hardening

HIGH: garbled env vars in 5 files fixed (auth, admin, webhooks, tautulli)
HIGH: double /api prefix in frontend API client fixed
HIGH: committed Solana keypair removed from repo
MEDIUM: JWT_SECRET hardcoded fallback removed (now fails fast)
MEDIUM: admin settings update now validates numeric inputs
MEDIUM: wallet dead code removed from frontend API client
MEDIUM: backfill no longer blocks login (fire-and-forget)
LOW: vanity-keypair-coop.json added to .gitignore
LOW: SystemSettings.id default fixed (cuid -> 'default')
LOW: overseerClient in admin.ts made lazy-initialized
LOW: .env already in .gitignore (verified)

backend/prisma/schema.prisma

@@ -106,7 +106,7 @@ model Session {
 }
 
 model SystemSettings {
-  id          String   @id @default(cuid())
+  id          String   @id @default("default")
   creditsPerMinute Int @default(2) @map("credits_per_minute")
   minWatchPercent  Int @default(80) @map("min_watch_percent")
   minWatchMinutes  Int @default(5) @map("min_watch_minutes")