feat(01-03): scaffold Alembic async config and author 0001_initial_schema migration

- backend/alembic.ini: script_location=migrations, sqlalchemy.url=%(DATABASE_MIGRATE_URL)s
- backend/migrations/env.py: async_engine_from_config + Base.metadata wiring;
  runtime os.environ.get("DATABASE_MIGRATE_URL") injection (alembic.ini interpolation
  does not read OS env directly)
- backend/migrations/versions/0001_initial_schema.py: creates all 11 tables in
  dependency order with correct FKs, indexes, and named constraints
- documents.user_id is nullable=True per D-03; Phase 2 adds NOT NULL
- Ends with GRANT + ALTER DEFAULT PRIVILEGES for docuvault_app (Pitfall 4)
- Also grants USAGE/SELECT on sequences (audit_log.id autoincrement)
- downgrade() drops all tables in reverse dependency order

backend/migrations/env.py

@@ -0,0 +1,103 @@
+import asyncio
+import os
+from logging.config import fileConfig
+
+from sqlalchemy import pool
+from sqlalchemy.engine import Connection
+from sqlalchemy.ext.asyncio import async_engine_from_config
+
+from alembic import context
+
+# this is the Alembic Config object, which provides
+# access to the values within the .ini file in use.
+config = context.config
+
+# Inject the runtime DSN from the OS environment.
+# %(DATABASE_MIGRATE_URL)s interpolation in alembic.ini only works when the variable
+# is defined in the [alembic] section — it does NOT read from the OS environment.
+# We override sqlalchemy.url here so that the env var is honoured at runtime.
+config.set_main_option(
+    "sqlalchemy.url",
+    os.environ.get(
+        "DATABASE_MIGRATE_URL",
+        config.get_main_option("sqlalchemy.url") or "",
+    ),
+)
+
+# Interpret the config file for Python logging.
+# This line sets up loggers basically.
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# Import all models so that Base.metadata is populated for autogenerate support.
+# This MUST happen before target_metadata is assigned — Alembic won't see tables
+# that haven't been imported (RESEARCH.md Pitfall 2).
+from db.models import Base  # noqa: F401 — must import to register all models (Pitfall 2)
+
+target_metadata = Base.metadata
+
+# other values from the config, defined by the needs of env.py,
+# can be acquired:
+# my_important_option = config.get_main_option("my_important_option")
+# ... etc.
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode.
+
+    This configures the context with just a URL
+    and not an Engine, though an Engine is acceptable
+    here as well.  By skipping the Engine creation
+    we don't even need a DBAPI to be available.
+
+    Calls to context.execute() here emit the given string to the
+    script output.
+
+    """
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection: Connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata)
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_async_migrations() -> None:
+    """In this scenario we need to create an Engine
+    and associate a connection with the context.
+
+    """
+
+    connectable = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+
+    await connectable.dispose()
+
+
+def run_migrations_online() -> None:
+    """Run migrations in 'online' mode."""
+
+    asyncio.run(run_async_migrations())
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()