Remove Google and GitHub sign-in

Both providers were registered but never usable: all four client
env vars are empty in production and no Account row has ever been
created. Email and password is the only path that has ever worked.

This also simplifies the invite gating that follows. With OAuth there
were two redemption paths, an invite token that had to survive the
provider round trip in a SameSite=Lax cookie, and an
OAuthAccountNotLinked dead end for anyone who signed up with a password
and later clicked a provider button. Now there is one path.

Account, Session and VerificationToken stay in the schema. They are
Auth.js's tables and dropping them would be a destructive migration for
no benefit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

install.sh

@@ -255,12 +255,6 @@ if [[ "$SKIP_CONFIG" == "false" ]]; then
   echo -e "${BOLD}AI Gateway:${NC}"
   prompt_value  SWITCHBOARD_URL "Switchboard gateway URL" "http://192.168.2.11:8787/v1"
 
-  echo ""
-  echo -e "${BOLD}OAuth Providers (optional — press Enter to skip):${NC}"
-  prompt_value  GOOGLE_CID    "Google Client ID"      ""
-  prompt_value  GOOGLE_CS     "Google Client Secret"  ""
-  prompt_value  GITHUB_CID    "GitHub Client ID"      ""
-  prompt_value  GITHUB_CS     "GitHub Client Secret"  ""
 
   # Build DATABASE_URL
   DATABASE_URL="postgresql://${PG_USER}:${PG_PASSWORD}@localhost:5432/${PG_DB}"
@@ -282,11 +276,6 @@ NEXTAUTH_URL="${APP_URL}"
 NEXTAUTH_SECRET="${AUTH_SECRET}"
 AUTH_TRUST_HOST="true"
 
-# ─── OAuth Providers ─────────────────────────────────────
-GOOGLE_CLIENT_ID="${GOOGLE_CID}"
-GOOGLE_CLIENT_SECRET="${GOOGLE_CS}"
-GITHUB_CLIENT_ID="${GITHUB_CID}"
-GITHUB_CLIENT_SECRET="${GITHUB_CS}"
 
 # ─── MinIO / S3-compatible storage ───────────────────────
 MINIO_ENDPOINT="localhost"
@@ -418,7 +407,4 @@ echo -e "  ${BOLD}${YELLOW}Next steps:${NC}"
 echo "    1. Open ${APP_URL_CHECK} and create your account"
 echo "    2. Add your Switchboard API key in Settings"
 echo "    3. Set up a reverse proxy (nginx/Caddy) for HTTPS"
-if [[ -z "${GOOGLE_CID:-}" ]] && [[ -z "${GITHUB_CID:-}" ]]; then
-  echo "    4. (Optional) Add OAuth providers in $ENV_FILE"
-fi
 echo ""