clear old handles from db if migrated to new did

pkg/appview/db/queries.go

@@ -373,6 +373,12 @@ func GetUserByHandle(db DBTX, handle string) (*User, error) {
 // InsertUserIfNotExists inserts a user record only if it doesn't already exist.
 // Used by non-profile collections to avoid unnecessary writes during backfill.
 func InsertUserIfNotExists(db DBTX, user *User) error {
+	// Clear handle from any other DID that currently holds it.
+	// In ATProto, a handle belongs to exactly one DID at a time —
+	// if a new DID claims this handle, the old association is stale.
+	_, _ = db.Exec(`UPDATE users SET handle = did WHERE handle = ? AND did != ?`,
+		user.Handle, user.DID)
+
 	_, err := db.Exec(`
         INSERT INTO users (did, handle, pds_endpoint, avatar, last_seen)
         VALUES (?, ?, ?, ?, ?)
@@ -383,6 +389,12 @@ func InsertUserIfNotExists(db DBTX, user *User) error {
 
 // UpsertUser inserts or updates a user record
 func UpsertUser(db DBTX, user *User) error {
+	// Clear handle from any other DID that currently holds it.
+	// In ATProto, a handle belongs to exactly one DID at a time —
+	// if a new DID claims this handle, the old association is stale.
+	_, _ = db.Exec(`UPDATE users SET handle = did WHERE handle = ? AND did != ?`,
+		user.Handle, user.DID)
+
 	_, err := db.Exec(`
         INSERT INTO users (did, handle, pds_endpoint, avatar, last_seen)
         VALUES (?, ?, ?, ?, ?)
@@ -398,6 +410,12 @@ func UpsertUser(db DBTX, user *User) error {
 // UpsertUserIgnoreAvatar inserts or updates a user record, but preserves existing avatar on update
 // This is useful when avatar fetch fails, and we don't want to overwrite an existing avatar with empty string
 func UpsertUserIgnoreAvatar(db DBTX, user *User) error {
+	// Clear handle from any other DID that currently holds it.
+	// In ATProto, a handle belongs to exactly one DID at a time —
+	// if a new DID claims this handle, the old association is stale.
+	_, _ = db.Exec(`UPDATE users SET handle = did WHERE handle = ? AND did != ?`,
+		user.Handle, user.DID)
+
 	_, err := db.Exec(`
         INSERT INTO users (did, handle, pds_endpoint, avatar, last_seen)
         VALUES (?, ?, ?, ?, ?)