feat(tools): fleet-wide deployment history calendar (Phase 2)

The per-unit Gantt chart on /unit/{id} (Phase 1, v0.11.0) was scoped
to one unit's deployment timeline.  This adds the fleet-wide view as
a new entry under /tools.

What it shows
- 12-month calendar grid styled like the Job Planner (4 months per
  row, responsive down to single column on mobile).
- Each day cell shows up to 4 colored mini-bars — one per project
  that had ≥1 active UnitAssignment that day, color deterministically
  hashed from project_id.  Days with >4 active projects show "+N".
- KPI strip at the top: project count, distinct unit count, total
  assignment count in the window.
- Collapsible project legend: ordered by first-active date (which
  matches the deployment-history reading order), each row links to
  the project page, shows the assignment count.

Click-a-day side panel
- Click any populated day cell → slide-over panel from the right
- Groups by project, lists every (unit, location) active that day
- Per-deployment: unit link, location link, window dates, active /
  closed badge, "auto-backfilled" tag for metadata_backfill source
- Sources from a new GET /api/admin/deployment-history/day endpoint

Navigation
- Prev / Next month buttons shift the 12-month window by one month
- "Recent" button jumps back to default (12 months ending now)
- Default window is 11 months back from current month — operator
  sees the recent past on first load, not future emptiness

Files
- backend/services/deployment_history.py — data builder + day-detail
  helper.  Walks UnitAssignment windows, intersects with the 12-month
  range, computes per-project active-day sets.
- backend/routers/deployment_history.py — page route + day-detail JSON
  endpoint.  Wired into main.py.
- templates/admin/deployment_history.html — page + side-panel
- templates/tools.html — new card linking to the page

Phase 3 (deferred): drag-to-resize bars to retroactively adjust
assignment windows from inside the calendar; per-unit row view
(complement to the project-row view) for "where has unit X been across
all jobs"; horizontal scroll for >12-month windows.

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

templates/tools.html

@@ -68,6 +68,24 @@
         </div>
     </a>
 
+    <!-- Deployment History -->
+    <a href="/tools/deployment-history"
+       class="block bg-white dark:bg-slate-800 rounded-xl shadow-lg p-5 hover:shadow-xl transition-shadow border border-transparent hover:border-seismo-orange">
+        <div class="flex items-start gap-3">
+            <div class="w-10 h-10 rounded-lg bg-indigo-100 dark:bg-indigo-900/30 text-indigo-600 dark:text-indigo-400 flex items-center justify-center shrink-0">
+                <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 7V3m8 4V3m-9 8h10M5 21h14a2 2 0 002-2V7a2 2 0 00-2-2H5a2 2 0 00-2 2v12a2 2 0 002 2z"/>
+                </svg>
+            </div>
+            <div class="flex-1 min-w-0">
+                <h3 class="font-semibold text-gray-900 dark:text-white">Deployment History</h3>
+                <p class="text-sm text-gray-600 dark:text-gray-400 mt-1">
+                    12-month calendar of every unit assignment across every project. Visual bars per project per day; click a day for the full active-deployments list.
+                </p>
+            </div>
+        </div>
+    </a>
+
     <!-- Reports (per-project) -->
     <a href="/projects"
        class="block bg-white dark:bg-slate-800 rounded-xl shadow-lg p-5 hover:shadow-xl transition-shadow border border-transparent hover:border-seismo-orange">