fix(deployment-timeline): respect user timezone for display and edits

Deployment timestamps were stored correctly as UTC but rendered raw —
a 1:30 PM EDT swap displayed as "5:30" because the frontend sliced the
naive UTC ISO string straight to the screen.

Display side: deployment_timeline.py now converts every emitted
timestamp (starts_at, ends_at, event_overlay.peak_pvs_at and last_event)
through `utc_to_local()` using the user's configured timezone from
UserPreferences before serializing.  Frontend slice keeps working — it
just slices a local-time string now.

Write side (so the new edit / add-historical-assignment modals stay
consistent):
  - PATCH /api/projects/{pid}/assignments/{aid}
  - POST  /api/projects/{pid}/locations/{loc}/assign
both now interpret a *naive* assigned_at / assigned_until ISO string as
the user's local time and convert to UTC for storage via
`local_to_utc()`.  Explicit tz-aware strings ("...Z" or "...+00:00")
skip the conversion so programmatic callers that already speak UTC
keep working.

Verified live: BE13121's stored 2026-01-28 18:06:29 UTC now serializes
as 2026-01-28 13:06:29 in the timeline endpoint; PATCHing
"2026-01-28T13:06:29" round-trips back to the same UTC value.

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

backend/services/deployment_timeline.py

@@ -39,9 +39,35 @@ from backend.services.sfm_events import (
     _fetch_events_for_serial,
     _iso_utc,
 )
+from backend.utils.timezone import utc_to_local
 
 log = logging.getLogger("backend.services.deployment_timeline")
 
+
+def _iso_local(dt) -> Optional[str]:
+    """Serialize a datetime / ISO-string in the user's configured timezone.
+
+    The timeline frontend slices these strings to character 19 to produce
+    "YYYY-MM-DD HH:MM:SS" — no JS-side timezone conversion happens.  We
+    therefore emit *already-local* timestamps here so the displayed time
+    matches what the operator actually saw on the wall clock.
+
+    Accepts either a ``datetime`` (DB column) or an ISO ``str`` (SFM
+    response).  Returns ``None`` for ``None`` input.  Naive ISO strings
+    from SFM are interpreted as UTC.
+    """
+    if dt is None:
+        return None
+    if isinstance(dt, str):
+        try:
+            dt = datetime.fromisoformat(dt.replace("Z", "").replace(" ", "T"))
+        except ValueError:
+            return dt   # give up gracefully — emit whatever SFM sent
+    local = utc_to_local(dt)
+    if local is None:
+        return None
+    return local.replace(tzinfo=None).isoformat()
+
 # Don't emit synthetic gap entries shorter than this (seconds).  Avoids visual
 # clutter from a sub-second handoff during a swap workflow.
 _MIN_GAP_SECONDS = 24 * 3600  # 1 day
@@ -185,8 +211,8 @@ async def deployment_timeline_for_unit(
             overlays[a.id] = {
                 "event_count": len(events),
                 "peak_pvs":    peak,
-                "peak_pvs_at": peak_at,
-                "last_event":  last_ev,
+                "peak_pvs_at": _iso_local(peak_at),
+                "last_event":  _iso_local(last_ev),
             }
 
     # 4. Build entries.  Start by emitting assignment rows + gap rows between
@@ -202,8 +228,8 @@ async def deployment_timeline_for_unit(
 
         entry = {
             "kind":          "assignment",
-            "starts_at":     _iso_utc(a.assigned_at),
-            "ends_at":       _iso_utc(a.assigned_until),
+            "starts_at":     _iso_local(a.assigned_at),
+            "ends_at":       _iso_local(a.assigned_until),
             "duration_days": round(duration_days, 1) if duration_days is not None else None,
             "assignment_id": a.id,
             "location_id":   a.location_id,
@@ -227,8 +253,8 @@ async def deployment_timeline_for_unit(
             if gap_seconds >= _MIN_GAP_SECONDS:
                 entries.append({
                     "kind":          "gap",
-                    "starts_at":     _iso_utc(gap_start),
-                    "ends_at":       _iso_utc(gap_end),
+                    "starts_at":     _iso_local(gap_start),
+                    "ends_at":       _iso_local(gap_end),
                     "duration_days": round(gap_seconds / 86400, 1),
                     "context":       "between assignments",
                 })
@@ -241,7 +267,7 @@ async def deployment_timeline_for_unit(
             continue
         entries.append({
             "kind":          "state_change",
-            "starts_at":     _iso_utc(h.changed_at),
+            "starts_at":     _iso_local(h.changed_at),
             "ends_at":       None,
             "duration_days": None,
             "change_type":   h.change_type,