feat(25-00): create chat_files and chat_file_references DB schema + migrations
- Add chatFiles Drizzle schema with companyId, conversationId, messageId, filename, mimeType, sizeBytes, objectKey, sha256, source, category, projectId columns
- Add chatFileReferences Drizzle schema for cross-conversation file references
- Add 0053_create_chat_files.sql migration with all columns, FKs, and indexes
- Add 0054_create_chat_file_references.sql migration with cascade deletes
- Export both tables from schema/index.ts
- Update _journal.json with entries for idx 53 and 54