feat: Sprint A Wahab review batch — categories, property hierarchy, priority grouping

Items 1-5, 9, 11 from denya-wahab-feedback-s2:
- Category.show_in_form (alert-only flag): Gas Leak hidden from issue picker
  but kept urgent for SLA/alert and reporting; emergency quick path on the
  new-issue form creates urgent tickets via include_hidden categories.
- Seed: Aluminum/Glass, Carpentry, Mould & Damp (Medium default) maintenance
  categories; Lost Property renamed Missing Item (+ sub).
- One alembic migration: add show_in_form (backfill True, Gas Leak False) +
  data rename Lost Property -> Missing Item.
- Property -> Building -> Apartment cascade with searchable apartment combobox
  on the new-issue form and ticket list filters; /api/tickets/units gains
  building filter + /units/grouped variant; /api/tickets gains additive
  building/unit_id filters. apartment_mapping.json committed (deterministic).
- Group-by-priority toggle on /tickets (four sections + unknown bucket,
  age-sortable, composes with filters, URL deep links), FM dashboard active
  tickets, and CS dashboard priority card click-through.
- Tests: 13 new (category visibility, seed idempotency/sync, unit grouping,
  ticket building/unit filters).

app/models/category.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from sqlalchemy import ForeignKey, Integer, String
+from sqlalchemy import Boolean, ForeignKey, Integer, String, true
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.core.database import Base
@@ -28,6 +28,13 @@ class Category(Base):
         nullable=True,
         comment="urgent, high, medium, low",
     )
+    show_in_form: Mapped[bool] = mapped_column(
+        Boolean,
+        nullable=False,
+        default=True,
+        server_default=true(),
+        comment="Visible in the new-issue category picker (False = alert-only, e.g. Gas Leak)",
+    )
 
     # self-referencing relationship
     children: Mapped[list[Category]] = relationship("Category", back_populates="parent", cascade="all, delete-orphan")