fix: migrate AI chat system from Firebase/Firestore to Postgres

Firebase was not configured so every chat request crashed with
'Firebase Admin credentials not configured'.

- chat-mode-resolver.ts: read project phase from fs_projects (Postgres)
- chat-context.ts: load project data from fs_projects instead of Firestore
- /api/ai/conversation: store/retrieve conversations in chat_conversations
  Postgres table (created automatically on first use)
- /api/ai/chat: replace all Firestore reads/writes with Postgres queries
- v_ai_chat/page.tsx: replace Firebase client auth with useSession from
  next-auth/react; remove Firestore listeners, use REST API for project data

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

app/api/ai/conversation/route.ts

@@ -1,12 +1,20 @@
 import { NextResponse } from 'next/server';
-import { getAdminDb } from '@/lib/firebase/admin';
+import { query } from '@/lib/db-postgres';
+
+const ENSURE_TABLE = `
+  CREATE TABLE IF NOT EXISTS chat_conversations (
+    project_id text PRIMARY KEY,
+    messages    jsonb NOT NULL DEFAULT '[]',
+    updated_at  timestamptz NOT NULL DEFAULT NOW()
+  )
+`;
 
 type StoredMessageRole = 'user' | 'assistant';
 
 type ConversationMessage = {
   role: StoredMessageRole;
   content: string;
-  createdAt?: { _seconds: number; _nanoseconds: number };
+  createdAt?: string;
 };
 
 type ConversationResponse = {
@@ -19,36 +27,43 @@ export async function GET(request: Request) {
     const projectId = (url.searchParams.get('projectId') ?? '').trim();
 
     if (!projectId) {
-      return NextResponse.json(
-        { error: 'projectId is required' },
-        { status: 400 },
-      );
+      return NextResponse.json({ error: 'projectId is required' }, { status: 400 });
     }
 
-    const adminDb = getAdminDb();
-    const docRef = adminDb.collection('chat_conversations').doc(projectId);
-    const snapshot = await docRef.get();
+    await query(ENSURE_TABLE);
 
-    if (!snapshot.exists) {
-      const empty: ConversationResponse = { messages: [] };
-      return NextResponse.json(empty);
-    }
-
-    const data = snapshot.data() as { messages?: ConversationMessage[] };
-    const messages = Array.isArray(data.messages) ? data.messages : [];
+    const rows = await query<{ messages: ConversationMessage[] }>(
+      `SELECT messages FROM chat_conversations WHERE project_id = $1`,
+      [projectId]
+    );
 
+    const messages: ConversationMessage[] = rows[0]?.messages ?? [];
     const response: ConversationResponse = { messages };
     return NextResponse.json(response);
   } catch (error) {
-    console.error('[ai/conversation] Failed to load conversation', error);
-    return NextResponse.json(
-      {
-        error: 'Failed to load conversation',
-        details: error instanceof Error ? error.message : String(error),
-      },
-      { status: 500 },
-    );
+    console.error('[GET /api/ai/conversation] Error:', error);
+    return NextResponse.json({ messages: [] });
   }
 }
 
+export async function DELETE(request: Request) {
+  try {
+    const url = new URL(request.url);
+    const projectId = (url.searchParams.get('projectId') ?? '').trim();
 
+    if (!projectId) {
+      return NextResponse.json({ error: 'projectId is required' }, { status: 400 });
+    }
+
+    await query(ENSURE_TABLE);
+    await query(
+      `DELETE FROM chat_conversations WHERE project_id = $1`,
+      [projectId]
+    );
+
+    return NextResponse.json({ ok: true });
+  } catch (error) {
+    console.error('[DELETE /api/ai/conversation] Error:', error);
+    return NextResponse.json({ error: 'Failed to reset conversation' }, { status: 500 });
+  }
+}