feat(tools): add Gantt view tab to deployment-history page

The Calendar grid (day-cells with project bars) is great for seeing
which projects had activity on a given day, but bad for seeing how
long any single deployment lasted.  The Gantt view inverts that —
one row per project, horizontal bars per assignment window — so an
operator can read durations at a glance.

Service layer
- backend/services/deployment_history.py extends each project's
  payload with `bars`: a list of {unit_id, location_id, location_name,
  start, end, is_active, source} for every UnitAssignment clipped to
  the visible 12-month window.  Location names are batch-resolved.
  Same cost as before since the underlying assignment scan is the
  same; just additional data in the response.

Template
- Tab switcher at the top of /tools/deployment-history toggles
  between Calendar and Gantt views.  URL hash (#gantt) preserves the
  active view across month-nav (Prev / Next / Recent buttons within
  the Gantt view link to ?...#gantt to stay on the same tab).
- Gantt view is a plain SVG with:
    - Left 220px label gutter: project color dot + truncated name,
      whole row clickable → opens the project page
    - Right area: horizontal time axis with month gridlines + labels,
      "today" dashed orange line, one row per project
    - One bar per assignment in that row, colored by project, reduced
      opacity for closed assignments, blue outline for metadata-
      backfilled assignments, white tip on the right edge of active
      bars
    - Hover any bar → tooltip with unit + location + window
- Alternating row backgrounds for readability.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/services/deployment_history.py

@@ -138,14 +138,26 @@ def get_deployment_history_data(
         p.id: p for p in db.query(Project).filter(Project.id.in_(proj_ids)).all()
     } if proj_ids else {}
 
+    # Resolve location names in one batch query (used by the Gantt view
+    # for per-bar tooltips).
+    from backend.models import MonitoringLocation
+    loc_ids = {a.location_id for a in assignments}
+    loc_name_map = {
+        l.id: l.name for l in db.query(MonitoringLocation).filter(
+            MonitoringLocation.id.in_(loc_ids)
+        ).all()
+    } if loc_ids else {}
+
     # Compute "active days per project" by walking each assignment and
     # adding every day in its [start, end] ∩ [first_date, last_date].
     # O(N_assignments × avg_window_days); for a typical fleet this is
     # bounded (hundreds of assignments × hundreds of days = manageable).
+    # Also collect raw per-assignment bar data for the Gantt view.
     project_active_days: dict[str, set[date]] = {}
     project_first_active: dict[str, date] = {}
     project_last_active:  dict[str, date] = {}
     project_assignment_count: dict[str, int] = {}
+    project_bars: dict[str, list[dict]] = {}
     distinct_units: set[str] = set()
 
     for a in assignments:
@@ -169,6 +181,20 @@ def get_deployment_history_data(
         if prev_last is None or end > prev_last:
             project_last_active[a.project_id] = end
 
+        # Per-assignment bar data — used by the Gantt view's renderer.
+        # `is_active` reflects whether the assignment_until was still NULL
+        # at fetch time (open-ended deployment); the clipped `end` here
+        # is just for visual bar drawing.
+        project_bars.setdefault(a.project_id, []).append({
+            "unit_id":       a.unit_id,
+            "location_id":   a.location_id,
+            "location_name": loc_name_map.get(a.location_id, "(unknown location)"),
+            "start":         start.isoformat(),
+            "end":           end.isoformat(),
+            "is_active":     a.assigned_until is None,
+            "source":        a.source,
+        })
+
     # Build the projects array (sorted by first_active ascending so the
     # legend reads in deployment-order).
     projects_data = []
@@ -186,6 +212,7 @@ def get_deployment_history_data(
                 "assignment_count": project_assignment_count.get(pid, 0),
                 "first_active": project_first_active[pid].isoformat() if pid in project_first_active else None,
                 "last_active":  project_last_active[pid].isoformat()  if pid in project_last_active  else None,
+                "bars":         project_bars.get(pid, []),
             })
             continue
         projects_data.append({
@@ -197,6 +224,7 @@ def get_deployment_history_data(
             "assignment_count": project_assignment_count.get(pid, 0),
             "first_active":     project_first_active[pid].isoformat() if pid in project_first_active else None,
             "last_active":      project_last_active[pid].isoformat()  if pid in project_last_active  else None,
+            "bars":             project_bars.get(pid, []),
         })
 
     projects_data.sort(key=lambda p: (p["first_active"] or "9999", p["name"]))