feat: implement Instagram clone SocialPhoto API

- FastAPI backend with SQLite database
- JWT authentication (register, login)
- User profiles with follow/unfollow
- Posts with image upload and likes/dislikes
- Comments with likes
- Global and personalized feed
- Comprehensive pytest test suite (37 tests)

TASK-ID: 758f4029-702

app/models.py

@@ -0,0 +1,109 @@
+"""Pydantic models for SocialPhoto API."""
+from datetime import datetime
+from typing import Optional, List
+
+from pydantic import BaseModel, ConfigDict, EmailStr, Field
+
+
+# Auth Models
+class UserRegister(BaseModel):
+    """Request model for user registration."""
+    username: str = Field(..., min_length=3, max_length=50)
+    email: EmailStr
+    password: str = Field(..., min_length=6)
+
+
+class UserLogin(BaseModel):
+    """Request model for user login."""
+    username: str
+    password: str
+
+
+class Token(BaseModel):
+    """Response model for JWT token."""
+    access_token: str
+    token_type: str = "bearer"
+
+
+# User Models
+class UserBase(BaseModel):
+    """Base user model."""
+    username: str
+    email: str
+    avatar_url: Optional[str] = "/static/default-avatar.png"
+    bio: Optional[str] = ""
+
+
+class UserResponse(UserBase):
+    """Response model for user data."""
+    id: int
+    created_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class UserStats(BaseModel):
+    """User statistics model."""
+    posts_count: int
+    followers_count: int
+    following_count: int
+
+
+# Post Models
+class PostCreate(BaseModel):
+    """Request model for creating a post."""
+    caption: Optional[str] = ""
+
+
+class PostResponse(BaseModel):
+    """Response model for post data."""
+    id: int
+    user_id: int
+    username: str
+    image_url: str
+    caption: str
+    likes_count: int
+    dislikes_count: int
+    comments_count: int
+    created_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+class PostDetail(PostResponse):
+    """Detailed post response with user info."""
+    user: UserResponse
+
+
+# Comment Models
+class CommentCreate(BaseModel):
+    """Request model for creating a comment."""
+    content: str = Field(..., min_length=1, max_length=500)
+
+
+class CommentResponse(BaseModel):
+    """Response model for comment data."""
+    id: int
+    post_id: int
+    user_id: int
+    username: str
+    content: str
+    likes_count: int
+    created_at: datetime
+
+    model_config = ConfigDict(from_attributes=True)
+
+
+# Feed Models
+class FeedResponse(BaseModel):
+    """Response model for feed."""
+    posts: List[PostResponse]
+    total: int
+    limit: int
+    offset: int
+
+
+# Error Models
+class ErrorResponse(BaseModel):
+    """Standard error response."""
+    detail: str