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>

backend/main.py

@@ -109,6 +109,9 @@ app.include_router(watcher_manager.router)
 from backend.routers import admin_modules
 app.include_router(admin_modules.router)
 
+from backend.routers import deployment_history
+app.include_router(deployment_history.router)
+
 # Projects system routers
 app.include_router(projects.router)
 app.include_router(project_locations.router)