feat: implement comprehensive achievement system with 62 achievements

Adds a complete achievement system with gamification features across all user interactions.

**Database & Types:**
- Add UserAchievement model to track user progress and earned achievements
- Add achievement-related fields to User model (achievementPoints, currentStreak, lastStreakCheck)
- Add ACHIEVEMENT_UNLOCKED audit action type
- Define 62 achievements as TypeScript constants across 5 categories

**Achievement Categories:**
- Suggestions (15): First suggestion through ultimate curator milestones
- Likes (12): First like through legendary fan milestones
- Comments (12): First comment through review legend milestones
- Engagement (15): Login streaks and total activity tracking
- Reports (8): Valid reports and accuracy tracking

**Backend Implementation:**
- Create AchievementService with comprehensive checking logic
- Add achievement route with 6 API endpoints
- Integrate achievement checking into all user interaction points:
  - Suggestions (create + accept)
  - Likes (toggle)
  - Comments (all 6 media types)
  - Login streaks
  - Reports (profile + comment)
- Update UserService to include achievement points in profiles

**Frontend Implementation:**
- Create AchievementService for API communication
- Create achievements page showing all 62 achievements with:
  - Category filtering (All, Suggestions, Likes, Comments, Engagement, Reports)
  - Tier-based styling (Bronze, Silver, Gold, Platinum, Diamond)
  - Progress indicators for in-progress achievements
  - Earned date display for completed achievements
- Add "Recent Achievements" section to user profiles
- Add "🏆 Achievements" link to header navigation
- Only show "View All" link on own profile

**Technical Features:**
- Real-time achievement checking on user actions
- Progress tracking to avoid recalculation
- Points system for gamification
- Tier-based gradient styling
- Leaderboard-ready architecture

Resolves #48

Co-Authored-By: Hikari <hikari@nhcarrigan.com>

api/prisma/schema.prisma

@@ -207,6 +207,9 @@ model User {
   isVip         Boolean        @default(false)
   isMod         Boolean        @default(false)
   isStaff       Boolean        @default(false)
+  achievementPoints Int         @default(0)
+  currentStreak     Int         @default(0)
+  lastStreakCheck   DateTime?
   createdAt     DateTime       @default(now())
   updatedAt     DateTime       @updatedAt
   comments         Comment[]
@@ -218,6 +221,7 @@ model User {
   reportsReviewed       ProfileReport[] @relation("Reviewer")
   commentReportsMade    CommentReport[] @relation("CommentReporter")
   commentReportsReviewed CommentReport[] @relation("CommentReviewer")
+  userAchievements      UserAchievement[]
 
   @@index([slug], map: "User_slug_key")
 }
@@ -276,6 +280,7 @@ enum AuditAction {
   RATE_LIMIT_EXCEEDED
   CSRF_VALIDATION_FAILED
   UNAUTHORIZED_ACCESS
+  ACHIEVEMENT_UNLOCKED
 }
 
 enum AuditCategory {
@@ -400,3 +405,20 @@ model CommentReport {
   @@index([reporterId])
   @@index([status])
 }
+
+model UserAchievement {
+  id              String      @id @default(auto()) @map("_id") @db.ObjectId
+  userId          String      @db.ObjectId
+  user            User        @relation(fields: [userId], references: [id])
+  achievementKey  String
+  progress        Int         @default(0)
+  earned          Boolean     @default(false)
+  earnedAt        DateTime?
+  createdAt       DateTime    @default(now())
+  updatedAt       DateTime    @updatedAt
+
+  @@unique([userId, achievementKey])
+  @@index([userId])
+  @@index([achievementKey])
+  @@index([earned])
+}