Chemistry: add Stock Solution Batch API (create, track volume, expire/dispose)

[joshholl]

Epic: #171

User Story

As a home developer, I want to record the bulk batches I mix so that I can track how much I have left and when they expire.

Acceptance Criteria

• New chemical_batches table: id, chemical_product_id (FK), label, total_volume_ml, remaining_volume_ml, concentration, mixed_date, expiry_date (nullable override), status (ACTIVE/EXHAUSTED/DISPOSED), notes
• expiry_date defaults to mixed_date + product.default_shelf_life_days if not overridden; null if product has no shelf life (e.g. Water)
• POST /api/chemistry/batches — create batch
• GET /api/chemistry/batches — list (optional ?status= filter)
• PUT /api/chemistry/batches/:id — update
• POST /api/chemistry/batches/:id/draw — decrement remaining_volume_ml by specified amount (called when a working solution is created from this batch)
• PATCH /api/chemistry/batches/:id/status — mark EXHAUSTED or DISPOSED
• Volume draw blocked if remainingVolumeMl < requested amount
• Unit tests for ChemicalBatchService

Notes

• Water batch: volume and expiry are unbounded (remaining_volume_ml can be null for system chemicals)