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/models/raw_material.py

@@ -18,6 +18,9 @@ class RawMaterial(Base):
     unit_of_measure: Mapped[str] = mapped_column(String(64))
     kg_per_unit: Mapped[float] = mapped_column(Float)
     status: Mapped[str] = mapped_column(String(32), default="active")
+    # Manually-assigned grouping used to order ingredients in the Mix Calculator
+    # output (e.g. "Grains", "Additives"). Optional; uncategorised rows sort last.
+    category: Mapped[str | None] = mapped_column(String(128), nullable=True)
     # Decimal places this ingredient's required-kg is rounded to in the mix
     # calculator output. Set per-ingredient from the Ingredients Editor.
     rounding_decimals: Mapped[int] = mapped_column(Integer, default=2)