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/routers/deployment_history.py

@@ -0,0 +1,99 @@
+"""
+Fleet-wide deployment-history calendar — Phase 2 of the
+deployment-history visualisation work (Phase 1 is the per-unit Gantt
+on /unit/{id}).
+
+Renders all UnitAssignment windows across all projects on a 12-month
+calendar grid styled like the Job Planner.  Each day cell shows one
+mini-bar per project that had ≥1 active assignment that day.  Click a
+day → side panel with the (unit, location) pairs active.
+
+Routes:
+  GET  /tools/deployment-history              — HTML page
+  GET  /api/admin/deployment-history/day      — JSON list of deployments
+                                                 on a specific date (used
+                                                 by the day-detail panel)
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime
+from typing import Optional
+
+from fastapi import APIRouter, Depends, Query, Request
+from fastapi.responses import HTMLResponse, JSONResponse
+from sqlalchemy.orm import Session
+
+from backend.database import get_db
+from backend.services.deployment_history import (
+    get_deployment_history_data,
+    get_deployments_on_day,
+)
+from backend.templates_config import templates
+
+router = APIRouter()
+
+
+@router.get("/tools/deployment-history", response_class=HTMLResponse)
+def deployment_history_page(
+    request: Request,
+    year:  Optional[int] = Query(None),
+    month: Optional[int] = Query(None),
+    db: Session = Depends(get_db),
+):
+    """Fleet-wide deployment history calendar.
+
+    Defaults to a 12-month window ending in the current month (so the
+    operator sees the recent past, not the future).  ?year=&month= can
+    override the START of the window to scroll backward or forward.
+    """
+    today = date.today()
+    # Default: 12-month window ending this month → start = 11 months back.
+    if year is None or month is None:
+        # 11 months back from current month.
+        m = today.month - 11
+        y = today.year
+        while m < 1:
+            m += 12
+            y -= 1
+        start_year, start_month = y, m
+    else:
+        start_year, start_month = year, month
+
+    calendar = get_deployment_history_data(db, start_year, start_month)
+
+    # Build prev/next navigation values.
+    prev_y, prev_m = (start_year - 1, 12) if start_month == 1 else (start_year, start_month - 1)
+    next_y, next_m = (start_year + 1, 1) if start_month == 12 else (start_year, start_month + 1)
+
+    return templates.TemplateResponse("admin/deployment_history.html", {
+        "request":  request,
+        "calendar": calendar,
+        "today":    today.isoformat(),
+        "prev_year":  prev_y,
+        "prev_month": prev_m,
+        "next_year":  next_y,
+        "next_month": next_m,
+    })
+
+
+@router.get("/api/admin/deployment-history/day")
+def deployment_history_day(
+    target_date: str = Query(..., description="YYYY-MM-DD"),
+    db: Session = Depends(get_db),
+):
+    """Return assignments active on a specific calendar day."""
+    try:
+        d = date.fromisoformat(target_date)
+    except ValueError:
+        return JSONResponse(
+            {"error": f"Invalid date: {target_date!r}"},
+            status_code=400,
+        )
+
+    deployments = get_deployments_on_day(db, d)
+    return JSONResponse({
+        "date":        target_date,
+        "count":       len(deployments),
+        "deployments": deployments,
+    })