Implement AI chat history system with enhanced memory (Phases 1 & 2)

Phase 1 - Database Schema & Basic API:
- Add ChatConversation and ChatMessage tables with proper relationships
- Create conversation CRUD API endpoints with authentication
- Update chat API to support persistent conversations
- Implement auto-generated conversation titles and language separation
- Add conversation soft delete and user-specific access control

Phase 2 - Enhanced Memory System:
- Implement intelligent context selection beyond simple chronological order
- Add relevance scoring for older messages based on keyword overlap and biblical references
- Create automatic message summarization for very long conversations
- Optimize token usage with smart context management (1500 token budget)
- Add biblical awareness with Romanian/English book name detection
- Implement time-based relevance decay for better context prioritization

Frontend Improvements:
- Add chat history button to floating chat header
- Create basic history panel UI with placeholder content
- Maintain backward compatibility for anonymous users

Database Changes:
- Enhanced schema with conversation relationships and message roles
- Proper indexing for performance and user-specific queries
- Migration completed successfully with prisma db push

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

prisma/schema.prisma

@@ -24,6 +24,7 @@ model User {
   chapterBookmarks ChapterBookmark[]
   notes           Note[]
   chatMessages    ChatMessage[]
+  chatConversations ChatConversation[]
   prayerRequests  PrayerRequest[]
   userPrayers     UserPrayer[]
   readingHistory  ReadingHistory[]
@@ -126,17 +127,43 @@ model BiblePassage {
   @@index([testament])
 }
 
+model ChatConversation {
+  id            String   @id @default(uuid())
+  userId        String?  // Optional for anonymous users
+  title         String   // Auto-generated from first message
+  language      String   // 'ro' or 'en'
+  isActive      Boolean  @default(true)
+  createdAt     DateTime @default(now())
+  updatedAt     DateTime @updatedAt
+  lastMessageAt DateTime @default(now())
+
+  user          User?    @relation(fields: [userId], references: [id], onDelete: Cascade)
+  messages      ChatMessage[]
+
+  @@index([userId, language, lastMessageAt])
+  @@index([isActive, lastMessageAt])
+}
+
 model ChatMessage {
-  id          String   @id @default(uuid())
-  userId      String
-  role        String   // 'user' or 'assistant'
-  content     String   @db.Text
-  metadata    Json?    // Store verse references, etc.
-  createdAt   DateTime @default(now())
+  id             String           @id @default(uuid())
+  conversationId String
+  userId         String?          // Keep for backward compatibility
+  role           ChatMessageRole
+  content        String           @db.Text
+  metadata       Json?            // Store verse references, etc.
+  timestamp      DateTime         @default(now())
 
-  user        User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  conversation   ChatConversation @relation(fields: [conversationId], references: [id], onDelete: Cascade)
+  user           User?            @relation(fields: [userId], references: [id], onDelete: Cascade)
 
-  @@index([userId, createdAt])
+  @@index([conversationId, timestamp])
+  @@index([userId, timestamp])
+}
+
+enum ChatMessageRole {
+  USER
+  ASSISTANT
+  SYSTEM
 }
 
 model Bookmark {