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>

.env.example

@@ -16,8 +16,12 @@ AZURE_OPENAI_API_VERSION=2024-02-15-preview
 # Ollama (optional)
 OLLAMA_API_URL=http://your-ollama-server:11434
 
-# Stripe (for donations)
+# Stripe (for donations & subscriptions)
 STRIPE_SECRET_KEY=sk_test_your_secret_key_here
 STRIPE_PUBLISHABLE_KEY=pk_test_your_publishable_key_here
 STRIPE_WEBHOOK_SECRET=whsec_your_webhook_secret_here
-NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=pk_test_your_publishable_key_here
+NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=pk_test_your_publishable_key_here
+
+# Stripe Subscription Price IDs (create these in Stripe Dashboard)
+STRIPE_PREMIUM_MONTHLY_PRICE_ID=price_xxxxxxxxxxxxx
+STRIPE_PREMIUM_YEARLY_PRICE_ID=price_xxxxxxxxxxxxx