feat: implement subscription system with conversation limits

Implement complete backend subscription system that limits free users to 10
AI conversations per month and offers Premium tier ($10/month or $100/year)
with unlimited conversations.

Changes:
- Add User subscription fields (tier, status, limits, counters)
- Create Subscription model to track Stripe subscriptions
- Implement conversation limit enforcement in chat API
- Add subscription checkout and customer portal APIs
- Update Stripe webhook to handle subscription events
- Add subscription utility functions (limit checks, tier management)
- Add comprehensive subscription translations (en, ro, es, it)
- Update environment variables for Stripe price IDs
- Update footer "Sponsor Us" link to point to /donate
- Add "Sponsor Us" button to home page hero section

Database:
- User model: subscriptionTier, subscriptionStatus, conversationLimit,
  conversationCount, limitResetDate, stripeCustomerId, stripeSubscriptionId
- Subscription model: tracks Stripe subscription details, periods, status
- SubscriptionStatus enum: ACTIVE, CANCELLED, PAST_DUE, TRIALING, etc.

API Routes:
- POST /api/subscriptions/checkout - Create Stripe checkout session
- POST /api/subscriptions/portal - Get customer portal link
- Webhook handlers for: customer.subscription.created/updated/deleted,
  invoice.payment_succeeded/failed

Features:
- Free tier: 10 conversations/month with automatic monthly reset
- Premium tier: Unlimited conversations
- Automatic limit enforcement before conversation creation
- Returns LIMIT_REACHED error with upgrade URL when limit hit
- Stripe Customer Portal integration for subscription management
- Automatic tier upgrade/downgrade via webhooks

Documentation:
- SUBSCRIPTION_IMPLEMENTATION_PLAN.md - Complete implementation plan
- SUBSCRIPTION_IMPLEMENTATION_STATUS.md - Current status and next steps

Frontend UI still needed: subscription page, upgrade modal, usage display

🤖 Generated with Claude Code (https://claude.com/claude-code)

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

app/api/chat/route.ts

@@ -3,6 +3,7 @@ import { z } from 'zod'
 import { PrismaClient, ChatMessageRole } from '@prisma/client'
 import { searchBibleHybrid, BibleVerse } from '@/lib/vector-search'
 import { verifyToken } from '@/lib/auth'
+import { checkConversationLimit, incrementConversationCount } from '@/lib/subscription-utils'
 
 const prisma = new PrismaClient()
 
@@ -57,6 +58,40 @@ export async function POST(request: Request) {
       )
     }
 
+    // Check conversation limits for new conversations only
+    if (userId && !conversationId) {
+      try {
+        const limitCheck = await checkConversationLimit(userId)
+
+        if (!limitCheck.allowed) {
+          return NextResponse.json(
+            {
+              success: false,
+              error: 'Conversation limit reached. Upgrade to Premium for unlimited conversations.',
+              code: 'LIMIT_REACHED',
+              data: {
+                limit: limitCheck.limit,
+                remaining: limitCheck.remaining,
+                tier: limitCheck.tier,
+                resetDate: limitCheck.resetDate,
+                upgradeUrl: `/${locale}/subscription`
+              }
+            },
+            { status: 403 }
+          )
+        }
+
+        console.log('Chat API - Limit check passed:', {
+          tier: limitCheck.tier,
+          remaining: limitCheck.remaining,
+          limit: limitCheck.limit
+        })
+      } catch (error) {
+        console.error('Chat API - Limit check error:', error)
+        // Allow the request to proceed if limit check fails
+      }
+    }
+
     // Handle conversation logic
     let finalConversationId = conversationId
     let conversationHistory: any[] = []
@@ -104,6 +139,15 @@ export async function POST(request: Request) {
           }
         })
         finalConversationId = conversation.id
+
+        // Increment conversation count for free tier users
+        try {
+          await incrementConversationCount(userId)
+          console.log('Chat API - Conversation count incremented for user:', userId)
+        } catch (error) {
+          console.error('Chat API - Failed to increment conversation count:', error)
+          // Continue anyway - don't block the conversation
+        }
       }
     } else {
       // Anonymous user - use provided history for backward compatibility