feat(dashboard): clarify the fleet status card and swap map locations to project monitoring location coords.

feat: Location no longer assigned directly to unit, locations and coords are assigned to location only, unit only is deployed or benched.

backend/services/unit_location.py

@@ -0,0 +1,125 @@
+"""
+Active-assignment location resolution for roster units.
+
+`RosterUnit.location`, `.address`, `.coordinates` are legacy per-unit fields.
+The current source of truth for "where is this unit deployed right now" is the
+active `UnitAssignment` (assigned_until IS NULL) pointing at a
+`MonitoringLocation`, which carries the canonical address/coordinates/name.
+
+Modems don't get their own `UnitAssignment` — they're paired with a
+seismograph or SLM via `deployed_with_unit_id`.  A deployed modem inherits the
+location of its paired device's active assignment.
+
+Returned dict shape (or None if no active assignment resolvable):
+    {
+        "location_id":   "uuid",
+        "project_id":    "uuid",
+        "name":          "NRL-001",
+        "address":       "123 Main St" | None,
+        "coordinates":   "34.0522,-118.2437" | None,
+        "via_paired_unit_id": "BE1234" | None,   # set only for modems
+    }
+"""
+
+from typing import Optional
+
+from sqlalchemy.orm import Session
+
+from backend.models import MonitoringLocation, RosterUnit, UnitAssignment
+
+
+def _serialize(loc: MonitoringLocation, via_paired_unit_id: Optional[str] = None) -> dict:
+    return {
+        "location_id": loc.id,
+        "project_id":  loc.project_id,
+        "name":        loc.name,
+        "address":     loc.address or None,
+        "coordinates": loc.coordinates or None,
+        "via_paired_unit_id": via_paired_unit_id,
+    }
+
+
+def _active_location_for_unit_id(db: Session, unit_id: str) -> Optional[MonitoringLocation]:
+    """Return the MonitoringLocation tied to this unit's active assignment, if any."""
+    row = (
+        db.query(MonitoringLocation)
+        .join(UnitAssignment, UnitAssignment.location_id == MonitoringLocation.id)
+        .filter(
+            UnitAssignment.unit_id == unit_id,
+            UnitAssignment.assigned_until == None,  # noqa: E711
+        )
+        .order_by(UnitAssignment.assigned_at.desc())
+        .first()
+    )
+    return row
+
+
+def get_active_location(db: Session, unit_id: str) -> Optional[dict]:
+    """
+    Resolve the active deployment location for a unit.
+
+    Seismographs / SLMs: their own active UnitAssignment.
+    Modems: follow `deployed_with_unit_id` to the paired device's active
+            assignment (modems don't carry their own assignment).
+    """
+    unit = db.query(RosterUnit).filter_by(id=unit_id).first()
+    if unit is None:
+        return None
+
+    if (unit.device_type or "seismograph") == "modem":
+        paired_id = unit.deployed_with_unit_id
+        if not paired_id:
+            return None
+        loc = _active_location_for_unit_id(db, paired_id)
+        return _serialize(loc, via_paired_unit_id=paired_id) if loc else None
+
+    loc = _active_location_for_unit_id(db, unit_id)
+    return _serialize(loc) if loc else None
+
+
+def bulk_active_locations(db: Session, units: list[RosterUnit]) -> dict[str, dict]:
+    """
+    Resolve active locations for many units in two queries.  Use this from
+    snapshot-style loops to avoid N+1 lookups.
+
+    Returns {unit_id: <serialized location dict>} — only populated for units
+    that resolve to an active assignment.  Modems are resolved by walking
+    `deployed_with_unit_id` to the paired device's entry in the same map.
+    """
+    if not units:
+        return {}
+
+    direct_unit_ids = [
+        u.id for u in units
+        if (u.device_type or "seismograph") != "modem"
+    ]
+
+    direct: dict[str, MonitoringLocation] = {}
+    if direct_unit_ids:
+        rows = (
+            db.query(UnitAssignment.unit_id, MonitoringLocation)
+            .join(MonitoringLocation, MonitoringLocation.id == UnitAssignment.location_id)
+            .filter(
+                UnitAssignment.unit_id.in_(direct_unit_ids),
+                UnitAssignment.assigned_until == None,  # noqa: E711
+            )
+            .order_by(UnitAssignment.assigned_at.desc())
+            .all()
+        )
+        # First row wins per unit_id (most recent assigned_at).
+        for unit_id, loc in rows:
+            direct.setdefault(unit_id, loc)
+
+    out: dict[str, dict] = {
+        uid: _serialize(loc) for uid, loc in direct.items()
+    }
+
+    # Modems inherit from paired device.
+    for u in units:
+        if (u.device_type or "seismograph") != "modem":
+            continue
+        paired_id = u.deployed_with_unit_id
+        if paired_id and paired_id in direct:
+            out[u.id] = _serialize(direct[paired_id], via_paired_unit_id=paired_id)
+
+    return out