feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

memento-note/prisma/schema.prisma

@@ -22,6 +22,7 @@ model User {
   createdAt             DateTime                @default(now())
   updatedAt             DateTime                @updatedAt
   cardSizeMode          String                  @default("variable")
+  accentColor           String                  @default("#A47148")
   accounts              Account[]
   agents                Agent[]
   aiFeedback            AiFeedback[]
@@ -43,6 +44,9 @@ model User {
   sessions              Session[]
   aiSettings            UserAISettings?
   workflows             Workflow[]
+  subscription          Subscription?
+  usageLogs             UsageLog[]
+  apiKeys               UserAPIKey[]
 }
 
 model Account {
@@ -256,6 +260,7 @@ model AiFeedback {
   @@index([noteId])
   @@index([userId])
   @@index([feature])
+  @@index([createdAt])
 }
 
 model MemoryEchoInsight {
@@ -630,3 +635,95 @@ model DocumentChunk {
   @@index([attachmentId])
   @@index([attachmentId, chunkIndex])
 }
+
+// ===== BYOK (Story 3.5) =====
+
+model UserAPIKey {
+  id           String   @id @default(cuid())
+  userId       String
+  provider     String
+  alias        String   @default("")
+  encryptedKey String
+  keyHash      String
+  model        String?
+  isActive     Boolean  @default(true)
+  lastUsedAt   DateTime?
+  lastUsedFor  String?
+  createdAt    DateTime @default(now())
+  updatedAt    DateTime @updatedAt
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, provider])
+  @@index([userId])
+  @@index([keyHash])
+}
+
+// ===== SUBSCRIPTION MODELS =====
+
+enum SubscriptionTier {
+  BASIC
+  PRO
+  BUSINESS
+  ENTERPRISE
+}
+
+enum SubscriptionStatus {
+  ACTIVE
+  PAST_DUE
+  CANCELED
+  TRIALING
+  INACTIVE
+}
+
+model Subscription {
+  id                    String   @id @default(cuid())
+  userId                String   @unique
+  tier                  SubscriptionTier @default(BASIC)
+  status                SubscriptionStatus @default(ACTIVE)
+
+  stripeCustomerId      String?  @unique
+  stripeSubscriptionId  String?  @unique
+  stripePriceId         String?
+
+  trialEndsAt           DateTime?
+  currentPeriodStart    DateTime
+  currentPeriodEnd      DateTime
+
+  canceledAt            DateTime?
+  cancelAtPeriodEnd     Boolean  @default(false)
+
+  createdAt             DateTime @default(now())
+  updatedAt             DateTime @updatedAt
+
+  user                  User @relation(fields: [userId], references: [id], onDelete: Cascade)
+}
+
+model UsageLog {
+  id             String   @id @default(cuid())
+  userId         String
+  feature        String
+  periodStart    DateTime
+  periodEnd      DateTime
+  requestsCount  Int      @default(0)
+  tokensUsed     Int      @default(0)
+
+  syncedAt       DateTime?
+  metadata       String?
+  createdAt      DateTime @default(now())
+
+  user           User @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, feature, periodStart])
+  @@index([userId, periodStart])
+  @@index([periodStart])
+}
+
+model FeatureFlag {
+  id        String   @id @default(cuid())
+  key       String   @unique
+  enabled   Boolean  @default(false)
+  tiers     String[]
+  metadata  String?
+  updatedAt DateTime @updatedAt
+}