Add local Pinscope multi-user auth for shared projects.

Self-host email/password accounts enable the existing collaborator
invite flow without Clerk; first admin inherits users/local projects.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/.env.example

@@ -52,7 +52,18 @@ GCS_BUCKET=
 # Frontend URL(s), JSON list
 CORS_ORIGINS=["http://localhost:3000","http://127.0.0.1:3000","http://localhost:18742","http://127.0.0.1:18742"]
 
-# -- DigiKey (optional) ------------------------------------------------------
+# -- Auth (self-host) --------------------------------------------------------
+# Set AUTH_JWT_SECRET to enable Pinscope email/password accounts and
+# multi-user project collaborators (invite by email). Clerk keys, if set,
+# take priority over local auth.
+# AUTH_JWT_SECRET=change-me-to-a-long-random-string
+# Comma-separated emails that are admin on register (first user is always admin)
+# AUTH_ADMIN_EMAILS=you@example.com
+
+# -- Clerk (optional cloud auth) ---------------------------------------------
+# CLERK_SECRET_KEY=
+# CLERK_PUBLISHABLE_KEY=
+# CLERK_JWKS_URL=
 # Optional catalog datasheet source and parameter-based passive auto-resolve.
 # Order: BOM URL → LCSC → manufacturer PDF URLs → Mouser → DigiKey.
 # DIGIKEY_CLIENT_ID=