feat: add twitter_handle to profiles with case-insensitive uniqueness

backend/migrations/005_add_twitter_handle.sql

@@ -0,0 +1,2 @@
+ALTER TABLE profiles ADD COLUMN IF NOT EXISTS twitter_handle TEXT;
+CREATE UNIQUE INDEX IF NOT EXISTS unique_twitter_handle_lower ON profiles (LOWER(twitter_handle));

backend/src/application/commands/create_profile.rs

@@ -35,6 +35,7 @@ pub async fn create_profile(
         description: profile.description,
         avatar_url: profile.avatar_url,
         github_login: profile.github_login,
+        twitter_handle: profile.twitter_handle,
         created_at: profile.created_at,
         updated_at: profile.updated_at,
     })

backend/src/application/commands/update_profile.rs

@@ -43,6 +43,31 @@ pub async fn update_profile(
             profile.github_login = Some(trimmed.to_string());
         }
     }
+    if let Some(ref handle) = request.twitter_handle {
+        let trimmed = handle.trim();
+
+        // Allow empty handles (set to None)
+        if trimmed.is_empty() {
+            profile.twitter_handle = None;
+        } else {
+            // Validate format for non-empty handles (Twitter/X handle: 1-15 alphanumeric + underscores)
+            let valid_format = regex::Regex::new(r"^[a-zA-Z0-9_]{1,15}$").unwrap();
+            if !valid_format.is_match(trimmed) {
+                return Err("Invalid Twitter handle format".to_string());
+            }
+            if let Some(conflicting_profile) = profile_repository
+                .find_by_twitter_handle(trimmed)
+                .await
+                .map_err(|e| e.to_string())?
+            {
+                // Only conflict if it's not the current user's profile
+                if conflicting_profile.address != wallet_address {
+                    return Err("Twitter handle already taken".to_string());
+                }
+            }
+            profile.twitter_handle = Some(trimmed.to_string());
+        }
+    }
     profile_repository
         .update(&profile)
         .await
@@ -54,6 +79,7 @@ pub async fn update_profile(
         description: profile.description,
         avatar_url: profile.avatar_url,
         github_login: profile.github_login,
+        twitter_handle: profile.twitter_handle,
         created_at: profile.created_at,
         updated_at: profile.updated_at,
     })

backend/src/application/dtos/profile_dtos.rs

@@ -15,6 +15,7 @@ pub struct UpdateProfileRequest {
     pub description: Option<String>,
     pub avatar_url: Option<String>,
     pub github_login: Option<String>,
+    pub twitter_handle: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -24,6 +25,7 @@ pub struct ProfileResponse {
     pub description: Option<String>,
     pub avatar_url: Option<String>,
     pub github_login: Option<String>,
+    pub twitter_handle: Option<String>,
     pub created_at: chrono::DateTime<chrono::Utc>,
     pub updated_at: chrono::DateTime<chrono::Utc>,
 }

backend/src/application/queries/get_all_profiles.rs

@@ -18,6 +18,7 @@ pub async fn get_all_profiles(
             description: profile.description,
             avatar_url: profile.avatar_url,
             github_login: profile.github_login,
+            twitter_handle: profile.twitter_handle,
             created_at: profile.created_at,
             updated_at: profile.updated_at,
         })

backend/src/application/queries/get_profile.rs

@@ -21,6 +21,7 @@ pub async fn get_profile(
         description: profile.description,
         avatar_url: profile.avatar_url,
         github_login: profile.github_login,
+        twitter_handle: profile.twitter_handle,
         created_at: profile.created_at,
         updated_at: profile.updated_at,
     })

backend/src/domain/entities/profile.rs

@@ -10,6 +10,7 @@ pub struct Profile {
     pub description: Option<String>,
     pub avatar_url: Option<String>,
     pub github_login: Option<String>,
+    pub twitter_handle: Option<String>,
     pub login_nonce: i64,
     pub created_at: DateTime<Utc>,
     pub updated_at: DateTime<Utc>,
@@ -24,6 +25,7 @@ impl Profile {
             description: None,
             avatar_url: None,
             github_login: None,
+            twitter_handle: None,
             login_nonce: 1,
             created_at: now,
             updated_at: now,

backend/src/domain/repositories/profile_repository.rs

@@ -16,6 +16,10 @@ pub trait ProfileRepository: Send + Sync {
         &self,
         github_login: &str,
     ) -> Result<Option<Profile>, Box<dyn std::error::Error + Send + Sync>>;
+    async fn find_by_twitter_handle(
+        &self,
+        twitter_handle: &str,
+    ) -> Result<Option<Profile>, Box<dyn std::error::Error + Send + Sync>>;
     async fn get_login_nonce_by_wallet_address(
         &self,
         address: &WalletAddress,