feat: merge cartsnitch/api into api/ subdirectory

Consolidate API gateway service into monorepo.
Squashed from https://github.com/cartsnitch/api main (89bacb1).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

alembic/versions/001_encrypt_session_data.py

@@ -0,0 +1,89 @@
+"""Encrypt existing plaintext session_data with Fernet.
+
+Revision ID: 001_encrypt_session_data
+Revises:
+Create Date: 2026-03-19
+"""
+
+import json
+import os
+
+import sqlalchemy as sa
+from cryptography.fernet import Fernet
+from sqlalchemy import text
+
+from alembic import op
+
+revision = "001_encrypt_session_data"
+down_revision = None
+branch_labels = None
+depends_on = None
+
+
+def _get_fernet() -> Fernet:
+    key = os.environ.get("CARTSNITCH_FERNET_KEY")
+    if not key:
+        raise RuntimeError("CARTSNITCH_FERNET_KEY must be set to run this migration")
+    return Fernet(key.encode())
+
+
+def _is_fernet_token(value: str) -> bool:
+    """Check if a string looks like a Fernet token (base64 starting with gAAAAA)."""
+    return value.startswith("gAAAAA")
+
+
+def upgrade() -> None:
+    # Change column type from JSON to TEXT to hold Fernet ciphertext
+    op.alter_column(
+        "user_store_accounts",
+        "session_data",
+        type_=sa.Text(),
+        existing_type=sa.JSON(),
+        existing_nullable=True,
+        postgresql_using="session_data::text",
+    )
+
+    conn = op.get_bind()
+    rows = conn.execute(
+        text("SELECT id, session_data FROM user_store_accounts WHERE session_data IS NOT NULL")
+    ).fetchall()
+
+    f = _get_fernet()
+    for row_id, session_data in rows:
+        raw = str(session_data)
+        if _is_fernet_token(raw):
+            continue
+        plaintext = raw if isinstance(session_data, str) else json.dumps(session_data)
+        encrypted = f.encrypt(plaintext.encode()).decode()
+        conn.execute(
+            text("UPDATE user_store_accounts SET session_data = :data WHERE id = :id"),
+            {"data": encrypted, "id": row_id},
+        )
+
+
+def downgrade() -> None:
+    conn = op.get_bind()
+    rows = conn.execute(
+        text("SELECT id, session_data FROM user_store_accounts WHERE session_data IS NOT NULL")
+    ).fetchall()
+
+    f = _get_fernet()
+    for row_id, session_data in rows:
+        raw = str(session_data)
+        if not _is_fernet_token(raw):
+            continue
+        decrypted = f.decrypt(raw.encode()).decode()
+        conn.execute(
+            text("UPDATE user_store_accounts SET session_data = :data WHERE id = :id"),
+            {"data": decrypted, "id": row_id},
+        )
+
+    # Revert column type from TEXT back to JSON
+    op.alter_column(
+        "user_store_accounts",
+        "session_data",
+        type_=sa.JSON(),
+        existing_type=sa.Text(),
+        existing_nullable=True,
+        postgresql_using="session_data::json",
+    )