Fix migrations 0004 and 0005 to be fully idempotent

The Dockerfile runs `alembic stamp --purge base && alembic upgrade head`
on every restart, so all migrations re-run against an existing database.
Migration 0004 used `op.add_column()` which fails on re-run with
"column already exists". Migration 0005 used `op.create_table()` which
would fail similarly.

Both now use raw SQL with idempotency guards:
- 0004: `DO $$ BEGIN ALTER TABLE ADD COLUMN ... EXCEPTION WHEN
  duplicate_column` + `IF EXISTS` check before touching the tasks table
- 0005: `CREATE TABLE IF NOT EXISTS` + `CREATE INDEX IF NOT EXISTS`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

alembic/versions/0005_add_chat_tables.py

@@ -15,54 +15,35 @@ depends_on = None
 
 
 def upgrade() -> None:
-    op.create_table(
-        "conversations",
-        sa.Column("id", sa.Integer(), primary_key=True),
-        sa.Column("title", sa.Text(), nullable=False, server_default=""),
-        sa.Column("model", sa.Text(), nullable=False, server_default=""),
-        sa.Column(
-            "created_at",
-            sa.DateTime(timezone=True),
-            server_default=sa.func.now(),
-            nullable=False,
-        ),
-        sa.Column(
-            "updated_at",
-            sa.DateTime(timezone=True),
-            server_default=sa.func.now(),
-            nullable=False,
-        ),
-    )
+    conn = op.get_bind()
 
-    op.create_table(
-        "messages",
-        sa.Column("id", sa.Integer(), primary_key=True),
-        sa.Column(
-            "conversation_id",
-            sa.Integer(),
-            sa.ForeignKey("conversations.id", ondelete="CASCADE"),
-            nullable=False,
-        ),
-        sa.Column("role", sa.Text(), nullable=False),
-        sa.Column("content", sa.Text(), nullable=False, server_default=""),
-        sa.Column(
-            "context_note_id",
-            sa.Integer(),
-            sa.ForeignKey("notes.id", ondelete="SET NULL"),
-            nullable=True,
-        ),
-        sa.Column(
-            "created_at",
-            sa.DateTime(timezone=True),
-            server_default=sa.func.now(),
-            nullable=False,
-        ),
-    )
+    conn.execute(sa.text("""
+        CREATE TABLE IF NOT EXISTS conversations (
+            id SERIAL PRIMARY KEY,
+            title TEXT NOT NULL DEFAULT '',
+            model TEXT NOT NULL DEFAULT '',
+            created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+            updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+        )
+    """))
 
-    op.create_index("ix_messages_conversation_id", "messages", ["conversation_id"])
+    conn.execute(sa.text("""
+        CREATE TABLE IF NOT EXISTS messages (
+            id SERIAL PRIMARY KEY,
+            conversation_id INTEGER NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
+            role TEXT NOT NULL,
+            content TEXT NOT NULL DEFAULT '',
+            context_note_id INTEGER REFERENCES notes(id) ON DELETE SET NULL,
+            created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+        )
+    """))
+
+    conn.execute(sa.text(
+        "CREATE INDEX IF NOT EXISTS ix_messages_conversation_id ON messages (conversation_id)"
+    ))
 
 
 def downgrade() -> None:
-    op.drop_index("ix_messages_conversation_id", table_name="messages")
-    op.drop_table("messages")
-    op.drop_table("conversations")
+    op.execute("DROP INDEX IF EXISTS ix_messages_conversation_id")
+    op.execute("DROP TABLE IF EXISTS messages")
+    op.execute("DROP TABLE IF EXISTS conversations")