feat(backend): add Alembic migrations

- Add alembic 1.14 to dependencies (uv sync → 1.18.4 installed)
- Configure alembic/env.py: loads DATABASE_URL from env, imports all
  SQLModel models so metadata is fully populated for autogenerate
- Generate initial migration (c2d626a2b36c) covering all 9 tables:
  products, product_inventory, medication_entries, medication_usages,
  lab_results, routines, routine_steps, grooming_schedule,
  skin_condition_snapshots — with all indexes and constraints
- Add ExecStartPre to innercontext.service: runs alembic upgrade head
  before uvicorn starts (idempotent, safe on every restart)
- Update DEPLOYMENT.md: add migration step to backend setup and update
  flow; document alembic stamp head for existing installations

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

backend/alembic/env.py

@@ -0,0 +1,50 @@
+import os
+from logging.config import fileConfig
+
+from alembic import context
+from dotenv import load_dotenv
+from sqlalchemy import engine_from_config, pool
+from sqlmodel import SQLModel
+
+load_dotenv()
+
+# Import all models so their tables are registered in SQLModel.metadata
+import innercontext.models  # noqa: F401
+
+config = context.config
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = SQLModel.metadata
+
+
+def get_url() -> str:
+    return os.environ.get("DATABASE_URL", "postgresql+psycopg://localhost/innercontext")
+
+
+def run_migrations_offline() -> None:
+    context.configure(
+        url=get_url(),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    cfg = config.get_section(config.config_ini_section, {})
+    cfg["sqlalchemy.url"] = get_url()
+    connectable = engine_from_config(cfg, 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()