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>

prisma/schema.prisma

@@ -20,6 +20,15 @@ model User {
   updatedAt             DateTime  @updatedAt
   lastLoginAt           DateTime?
 
+  // Subscription fields
+  subscriptionTier      String    @default("free") // "free", "premium"
+  subscriptionStatus    String    @default("active") // "active", "cancelled", "expired", "past_due"
+  conversationLimit     Int       @default(10)
+  conversationCount     Int       @default(0) // Reset monthly
+  limitResetDate        DateTime? // When to reset conversation count
+  stripeCustomerId      String?   @unique // For subscriptions (separate from donations)
+  stripeSubscriptionId  String?   @unique
+
   sessions        Session[]
   bookmarks       Bookmark[]
   chapterBookmarks ChapterBookmark[]
@@ -38,8 +47,11 @@ model User {
   updatedSocialMedia SocialMediaLink[] @relation("SocialMediaUpdater")
   updatedMailgunSettings MailgunSettings[] @relation("MailgunSettingsUpdater")
   donations          Donation[]
+  subscriptions      Subscription[]
 
   @@index([role])
+  @@index([subscriptionTier])
+  @@index([stripeCustomerId])
 }
 
 model Session {
@@ -455,3 +467,36 @@ enum DonationStatus {
   REFUNDED
   CANCELLED
 }
+
+model Subscription {
+  id                    String             @id @default(uuid())
+  userId                String
+  stripeSubscriptionId  String             @unique
+  stripePriceId         String             // Stripe price ID for the plan
+  stripeCustomerId      String
+  status                SubscriptionStatus
+  currentPeriodStart    DateTime
+  currentPeriodEnd      DateTime
+  cancelAtPeriodEnd     Boolean            @default(false)
+  tier                  String             // "premium"
+  interval              String             // "month" or "year"
+  metadata              Json?
+  createdAt             DateTime           @default(now())
+  updatedAt             DateTime           @updatedAt
+
+  user                  User               @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([status])
+  @@index([stripeSubscriptionId])
+}
+
+enum SubscriptionStatus {
+  ACTIVE
+  CANCELLED
+  PAST_DUE
+  TRIALING
+  INCOMPLETE
+  INCOMPLETE_EXPIRED
+  UNPAID
+}