Add bulk install-in-device from dashboard and unretire action

- Dashboard bulk toolbar: select batteries, pick a device, click 'Install
  in device'; confirms before moving already-installed batteries; enforces
  slot capacity and warns on brand mix
- Battery detail: 'Unretire Battery' button replaces 'Retire Battery' when
  battery is retired, restoring it to available status
- Tests: 3 new bulk-install-device tests (capacity block, move, success);
  42 total passing

app.py

@@ -225,6 +225,23 @@ def create_app(config_object="config"):
         flash(f"{battery.label} has been retired.", "success")
         return redirect(url_for("dashboard"))
 
+    # ------------------------------------------------------------------ #
+    # Battery — unretire
+    # ------------------------------------------------------------------ #
+
+    @app.route("/battery/<int:battery_id>/unretire", methods=["POST"])
+    def battery_unretire(battery_id):
+        battery = db.get(Battery, battery_id)
+        if battery is None:
+            abort(404)
+        if not battery.is_retired():
+            flash(f"{battery.label} is not retired.", "error")
+            return redirect(url_for("battery_detail", battery_id=battery_id))
+        battery.status = "available"
+        db.commit()
+        flash(f"{battery.label} is now available again.", "success")
+        return redirect(url_for("battery_detail", battery_id=battery_id))
+
     # ------------------------------------------------------------------ #
     # Battery — delete
     # ------------------------------------------------------------------ #
@@ -285,6 +302,55 @@ def create_app(config_object="config"):
                 b.brand = new_brand
             db.commit()
             flash(f"Updated brand to '{new_brand}' for {n} batter{'y' if n == 1 else 'ies'}.", "success")
+        elif action == "install_device":
+            device_id = request.form.get("device_id", type=int)
+            if not device_id:
+                flash("Please select a device.", "error")
+                return redirect(url_for("dashboard"))
+            device = db.get(Device, device_id)
+            if device is None:
+                flash("Device not found.", "error")
+                return redirect(url_for("dashboard"))
+
+            already_here = [b for b in batteries if b.device_id == device.id]
+            retired_sel  = [b for b in batteries if b.is_retired()]
+            to_process   = [b for b in batteries
+                            if b.device_id != device.id and not b.is_retired()]
+
+            if not to_process:
+                flash("No eligible batteries to install.", "error")
+                return redirect(url_for("dashboard"))
+
+            free_slots = device.battery_slots - device.installed_count()
+            if len(to_process) > free_slots:
+                flash(
+                    f"{device.name} only has {free_slots} free slot(s), "
+                    f"but {len(to_process)} need installing.",
+                    "error",
+                )
+                return redirect(url_for("dashboard"))
+
+            existing_brands = device.installed_brands()
+            new_brands = set(b.brand for b in to_process)
+            if existing_brands and (new_brands - existing_brands):
+                flash(f"Warning: mixing brands in {device.name}.", "warning")
+
+            for b in to_process:
+                b.status = "installed"
+                b.device_id = device.id
+            db.commit()
+
+            msg = (f"Installed {len(to_process)} "
+                   f"batter{'y' if len(to_process) == 1 else 'ies'} in {device.name}.")
+            notes = []
+            if already_here:
+                notes.append(f"{len(already_here)} already there")
+            if retired_sel:
+                notes.append(f"{len(retired_sel)} retired skipped")
+            if notes:
+                msg += f" ({', '.join(notes)}.)"
+            flash(msg, "success")
+
         elif action == "set_field":
             field_name = request.form.get("field_name", "").strip()
             field_value = request.form.get("field_value", "").strip() or None