fix(install): generate PLATFORM_URL in install.sh, backfill on existing .env

Fresh installs failed at 'docker compose up' with 'required variable
PLATFORM_URL is missing a value' — the install script computes ORIGIN_URL
for CORS_ORIGINS already but never wrote PLATFORM_URL, which the backend
now requires with no default. Also backfills it into an existing .env
(from CORS_ORIGINS) when the user chooses to keep their current config,
so upgrading an older install in place doesn't hit the same error.

scripts/install.sh

@@ -117,6 +117,17 @@ if [ -f "$ENV_FILE" ]; then
     if [[ ! $REPLY =~ ^[Yy]$ ]]; then
         print_info "Keeping existing configuration."
         SKIP_CONFIG=true
+
+        # Older installs predate PLATFORM_URL (added when email links moved
+        # off the dev default) — backfill it from CORS_ORIGINS so upgrading
+        # in place doesn't break the backend's required-env-var check.
+        if ! grep -q '^PLATFORM_URL=' "$ENV_FILE" 2>/dev/null; then
+            EXISTING_ORIGIN=$(grep '^CORS_ORIGINS=' "$ENV_FILE" 2>/dev/null | cut -d= -f2-)
+            if [ -n "$EXISTING_ORIGIN" ]; then
+                echo "PLATFORM_URL=${EXISTING_ORIGIN}" >> "$ENV_FILE"
+                print_info "Added missing PLATFORM_URL=${EXISTING_ORIGIN} to existing .env"
+            fi
+        fi
     fi
 fi
 
@@ -314,6 +325,10 @@ MINIO_BUCKET=evidence
 # ── CORS (allowed frontend origins) ─────────────────────────────────────────
 CORS_ORIGINS=${ORIGIN_URL}
 
+# ── Emails ────────────────────────────────────────────────────────────────────
+# Base URL used to build links in outbound emails (set-password, etc).
+PLATFORM_URL=${ORIGIN_URL}
+
 # ── Frontend ─────────────────────────────────────────────────────────────────
 FRONTEND_PORT=${FRONTEND_PORT}