Remove dead code: projectChat.ts and streaming route replaced by AgentCore

Deleted backend/src/routes/projectChat.ts entirely (POST /projects/:projectId/chat
streaming handler - frontend calls AgentCore directly). Removed its import and
app.use line from app.ts.

Removed the POST /chat/ streaming handler from chat.ts and its exclusive imports
(chatTools functions and checkCredits). Note: chatTools.ts is retained because
tabular.ts still actively uses runLLMStream, TABULAR_TOOLS, ChatMessage, and
TabularCellStore from it.