Fix PostgreSQL compatibility in migrations

- tasks.0002_initial: Remove duplicate user field on Tag model
- tasks.0005_fix_tasks_tags_table: Add PostgreSQL/MySQL/SQLite support
  - Use SERIAL instead of AUTOINCREMENT for PostgreSQL
  - Use UUID type for foreign keys in PostgreSQL
- tasks.0006_fix_task_shares_table: Add PostgreSQL/MySQL/SQLite support
  - Use UUID type and TIMESTAMP in PostgreSQL
  - Use CHAR(32) and DATETIME in MySQL/SQLite

These migrations now detect the database backend and use appropriate
syntax for each database type.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tasks/migrations/0005_fix_tasks_tags_table.py

@@ -10,11 +10,43 @@ def fix_tasks_tags_table(apps, schema_editor):
     """
     db_alias = schema_editor.connection.alias
 
+    # Check database type
+    is_postgres = schema_editor.connection.vendor == 'postgresql'
+    is_mysql = schema_editor.connection.vendor == 'mysql'
+
     # Drop and recreate the tasks_tags table with correct foreign keys
     schema_editor.execute(
-        "DROP TABLE IF EXISTS tasks_tags;"
+        "DROP TABLE IF EXISTS tasks_tags CASCADE;" if is_postgres else "DROP TABLE IF EXISTS tasks_tags;"
     )
 
+    if is_postgres:
+        # PostgreSQL syntax
+        schema_editor.execute(
+            """
+            CREATE TABLE tasks_tags (
+                id SERIAL PRIMARY KEY,
+                task_id UUID NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+                tag_id UUID NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+                UNIQUE (task_id, tag_id)
+            );
+            """
+        )
+    elif is_mysql:
+        # MySQL/MariaDB syntax
+        schema_editor.execute(
+            """
+            CREATE TABLE tasks_tags (
+                id INTEGER PRIMARY KEY AUTO_INCREMENT,
+                task_id CHAR(32) NOT NULL,
+                tag_id CHAR(32) NOT NULL,
+                UNIQUE (task_id, tag_id),
+                FOREIGN KEY (task_id) REFERENCES tasks(id) ON DELETE CASCADE,
+                FOREIGN KEY (tag_id) REFERENCES tags(id) ON DELETE CASCADE
+            );
+            """
+        )
+    else:
+        # SQLite syntax (default)
         schema_editor.execute(
             """
             CREATE TABLE tasks_tags (

tasks/migrations/0006_fix_task_shares_table.py

@@ -10,11 +10,63 @@ def fix_task_shares_table(apps, schema_editor):
     """
     db_alias = schema_editor.connection.alias
 
+    # Check database type
+    is_postgres = schema_editor.connection.vendor == 'postgresql'
+    is_mysql = schema_editor.connection.vendor == 'mysql'
+
     # Drop and recreate the task_shares table with correct foreign keys
     schema_editor.execute(
-        "DROP TABLE IF EXISTS task_shares;"
+        "DROP TABLE IF EXISTS task_shares CASCADE;" if is_postgres else "DROP TABLE IF EXISTS task_shares;"
     )
 
+    if is_postgres:
+        # PostgreSQL syntax
+        schema_editor.execute(
+            """
+            CREATE TABLE task_shares (
+                id UUID NOT NULL PRIMARY KEY,
+                permission VARCHAR(20) NOT NULL,
+                sync_id UUID NOT NULL UNIQUE,
+                created_at TIMESTAMP NOT NULL,
+                updated_at TIMESTAMP NOT NULL,
+                owner_id UUID NOT NULL REFERENCES users(id) DEFERRABLE INITIALLY DEFERRED,
+                shared_with_id UUID NOT NULL REFERENCES users(id) DEFERRABLE INITIALLY DEFERRED,
+                task_id UUID NULL REFERENCES tasks(id) ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED,
+                tag_id UUID NULL REFERENCES tags(id) ON DELETE CASCADE DEFERRABLE INITIALLY DEFERRED,
+                CONSTRAINT share_task_or_tag_not_both CHECK (
+                    (tag_id IS NULL AND task_id IS NOT NULL) OR
+                    (tag_id IS NOT NULL AND task_id IS NULL)
+                )
+            );
+            """
+        )
+    elif is_mysql:
+        # MySQL/MariaDB syntax
+        schema_editor.execute(
+            """
+            CREATE TABLE task_shares (
+                id CHAR(32) NOT NULL PRIMARY KEY,
+                permission VARCHAR(20) NOT NULL,
+                sync_id CHAR(32) NOT NULL UNIQUE,
+                created_at DATETIME NOT NULL,
+                updated_at DATETIME NOT NULL,
+                owner_id CHAR(32) NOT NULL,
+                shared_with_id CHAR(32) NOT NULL,
+                task_id CHAR(32) NULL,
+                tag_id CHAR(32) NULL,
+                CONSTRAINT share_task_or_tag_not_both CHECK (
+                    (tag_id IS NULL AND task_id IS NOT NULL) OR
+                    (tag_id IS NOT NULL AND task_id IS NULL)
+                ),
+                FOREIGN KEY (owner_id) REFERENCES users(id),
+                FOREIGN KEY (shared_with_id) REFERENCES users(id),
+                FOREIGN KEY (task_id) REFERENCES tasks(id) ON DELETE CASCADE,
+                FOREIGN KEY (tag_id) REFERENCES tags(id) ON DELETE CASCADE
+            );
+            """
+        )
+    else:
+        # SQLite syntax (default)
         schema_editor.execute(
             """
             CREATE TABLE task_shares (