feat(migrations): T11 setup Alembic and initial schema migration

- Initialize Alembic with alembic init alembic
- Configure alembic.ini to use DATABASE_URL from environment
- Configure alembic/env.py to import Base and models metadata
- Generate initial migration: c92fc544a483_initial_schema
- Migration creates all 4 tables: users, api_keys, api_tokens, usage_stats
- Migration includes all indexes, constraints, and foreign keys
- Test upgrade/downgrade cycle works correctly

Alembic commands:
- alembic upgrade head
- alembic downgrade -1
- alembic revision --autogenerate -m 'message'

Tests: 13 migration tests pass

alembic/README

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+Generic single-database configuration.

alembic/env.py

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+"""Alembic environment configuration.
+
+T11: Setup Alembic and initial schema migration
+"""
+import os
+import sys
+from logging.config import fileConfig
+
+from sqlalchemy import engine_from_config, pool
+
+from alembic import context
+
+# Add src to path to import models
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..', 'src'))
+
+# Import models to register them with Base
+from openrouter_monitor.database import Base
+from openrouter_monitor.models import User, ApiKey, UsageStats, ApiToken
+
+# this is the Alembic Config object, which provides
+# access to the values within the .ini file in use.
+config = context.config
+
+# Override sqlalchemy.url with environment variable if available
+# This allows DATABASE_URL from .env to be used
+database_url = os.getenv('DATABASE_URL')
+if database_url:
+    config.set_main_option('sqlalchemy.url', database_url)
+
+# 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)
+
+# Set target_metadata to the Base.metadata from our models
+# This is required for 'autogenerate' support
+target_metadata = Base.metadata
+
+
+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 run_migrations_online() -> None:
+    """Run migrations in 'online' mode.
+
+    In this scenario we need to create an Engine
+    and associate a connection with the context.
+
+    """
+    # For SQLite, we need to handle check_same_thread=False
+    db_url = config.get_main_option("sqlalchemy.url")
+    
+    if db_url and 'sqlite' in db_url:
+        # SQLite specific configuration
+        from sqlalchemy import create_engine
+        connectable = create_engine(
+            db_url,
+            connect_args={"check_same_thread": False},
+            poolclass=pool.NullPool,
+        )
+    else:
+        connectable = engine_from_config(
+            config.get_section(config.config_ini_section, {}),
+            prefix="sqlalchemy.",
+            poolclass=pool.NullPool,
+        )
+
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection, target_metadata=target_metadata
+        )
+
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

alembic/script.py.mako

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+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}