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>

docs/DEPLOYMENT.md

@@ -137,12 +137,25 @@ chmod 600 /opt/innercontext/backend/.env
 chown innercontext:innercontext /opt/innercontext/backend/.env
 ```
 
+### Run database migrations
+
+```bash
+sudo -u innercontext bash -c '
+  cd /opt/innercontext/backend
+  uv run alembic upgrade head
+'
+```
+
+This creates all tables on first run. On subsequent deploys it applies only the new migrations.
+
+> **Existing database (tables already created by `create_db_and_tables`):**
+> Run `uv run alembic stamp head` instead to mark the current schema as migrated without re-running DDL.
+
 ### Test
 
 ```bash
 sudo -u innercontext bash -c '
   cd /opt/innercontext/backend
-  source .env
   uv run uvicorn main:app --host 127.0.0.1 --port 8000
 '
 # Ctrl-C after confirming it starts
@@ -260,9 +273,12 @@ The web UI should be accessible at `http://innercontext.lan`.
 cd /opt/innercontext
 git pull
 
-# If backend dependencies changed:
+# Sync backend dependencies if pyproject.toml changed:
 cd backend && sudo -u innercontext uv sync && cd ..
 
+# Apply any new DB migrations (runs automatically via ExecStartPre, but safe to run manually first):
+sudo -u innercontext bash -c 'cd /opt/innercontext/backend && uv run alembic upgrade head'
+
 # Rebuild frontend:
 cd frontend && sudo -u innercontext bash -c '
   pnpm install