docs: replace TPB reference with DB_README and add database guide scaffold

DB_README.md

@@ -0,0 +1,44 @@
+# Database & Backend Notes
+
+This document is the central place for database-related setup, schema notes, and backend conventions for car64.
+
+## Stack
+
+- Supabase Postgres
+- Supabase Auth
+- Supabase Storage
+- Supabase RPC functions
+
+## Environments
+
+Document your environment endpoints and key handling policy here.
+
+## Schema Overview
+
+Document important tables and relations here, for example:
+
+- `collections`
+- `collection_members`
+- `hotwheels`
+- `global_cars`
+- `car_votes`
+- `car_reports`
+
+## RLS Policies
+
+Document Row Level Security rules and expected access behavior.
+
+## RPC Functions
+
+List all RPC functions and their purpose.
+
+## Migration Workflow
+
+Describe how migrations are authored, reviewed, and applied.
+
+## Operational Checklist
+
+- Verify schema changes in staging first
+- Confirm RLS policy impact
+- Validate app compatibility with query/RPC changes
+- Keep rollback strategy documented

README.md

@@ -192,5 +192,5 @@ See `LICENSE` for details.
 
 - Contributor guidelines: `CONTRIBUTING.md`
 - Security policy: `SECURITY.md`
-- Product/backend notes: `TPB.md`
+- Database and backend notes: `DB_README.md`