v0.1.32 - Mix Calculator search, ingredient categories, throughput tidy-up

- Mix Calculator: searchable Mix Name picker (mirrors Throughput search)
- Ingredients Editor: add manual Category column; used to order Mix Calculator output
- Mix Calculator: surface formula-only mixes (no product yet) via -mix_id sentinel
- Throughput: remove unused For order / For stock destination controls from composer
- Editor change history: show timestamps in local time (stored UTC) instead of raw UTC

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/db/migrations.py

@@ -139,6 +139,7 @@ _LEGACY_COLUMN_PATCHES: tuple[tuple[str, str, str], ...] = (
     ("production_throughput_entries", "job_number", "VARCHAR(64)"),
     ("production_throughput_entries", "stock_quantity", "FLOAT"),
     ("raw_materials", "rounding_decimals", "INTEGER NOT NULL DEFAULT 2"),
+    ("raw_materials", "category", "VARCHAR(128)"),
     ("mix_calculator_session_lines", "rounding_decimals", "INTEGER NOT NULL DEFAULT 2"),
 )