feat(deployments): field-capture endpoint for pending deployments (commit 1)

Field-install workflow needs to be fast: arrive on site, snap a photo
of the seismograph in place, leave.  Project / location classification
happens later at a desk.  This adds the data model + capture endpoint
for that workflow.

Data model
- New PendingDeployment table.  Lifecycle: awaiting → assigned (when
  promoted to a real UnitAssignment) or → cancelled (operator's
  mistake).  Photos are filesystem files under data/photos/{unit_id}/
  with the filename stored on the row.
- Migration: backend/migrate_add_pending_deployments.py (idempotent).

Endpoints
- POST /api/deployments/capture  — multipart upload (unit_id, photo,
  optional note).  Refuses non-seismographs.  Extracts EXIF GPS
  (cribbing extract_exif_data from routers/photos.py) and stores
  the captured "lat,lon" on the row.  Saves the photo under
  data/photos/{unit_id}/install_YYYYMMDD_HHMMSS_<uuid8>.<ext>.
  Returns the new pending_deployment_id + extracted coords + photo
  URL for the client to render confirmation.
- GET  /api/deployments/pending           — list by status (default awaiting)
- GET  /api/deployments/pending/{id}      — single row detail
- POST /api/deployments/pending/{id}/promote — classify → create
  UnitAssignment.  Body accepts two shapes: assign-to-existing-location
  OR create-new-location (with new-or-existing project).  Sets
  status=assigned, resulting_assignment_id, promoted_at.
- POST /api/deployments/pending/{id}/cancel  — abandon with optional reason.

All four routes write UnitHistory audit rows
(pending_deployment_captured / _promoted / _cancelled).

Events from a unit with an unclassified pending deployment land in the
unit's "Unattributed" events bucket as usual.  Once promoted, the new
UnitAssignment's window retroactively attributes them — same mechanism
the metadata-backfill tool uses.

Seismograph-only for v1.  SLM deployments don't follow this pattern
and are tracked elsewhere.  Capture refuses non-seismograph unit_ids
with HTTP 400.

UI (commits 2 + 3) lands next.

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

backend/models.py

@@ -644,3 +644,58 @@ class JobReservationUnit(Base):
     # Location identity
     location_name = Column(String, nullable=True)  # e.g. "North Gate", "Main Entrance"
     slot_index = Column(Integer, nullable=True)     # Order within reservation (0-based)
+
+
+class PendingDeployment(Base):
+    """
+    Field-captured "I just installed this seismograph" record waiting
+    to be classified into a project + location.
+
+    Lifecycle:
+      1. Operator captures from the /deploy mobile page — photo (EXIF
+         GPS auto-extracted), optional free-text note.  Row created
+         with status="awaiting".
+      2. Later, at a desk: operator picks a project + location (existing
+         or new) and "promotes" the row.  A real UnitAssignment is
+         created, this row's status flips to "assigned", and
+         resulting_assignment_id points at the new assignment.
+      3. Mistakes / abandoned captures → status="cancelled" with a
+         cancelled_reason for audit.
+
+    Events emitted by the unit before classification are NOT auto-
+    attributed (no UnitAssignment exists yet).  They land in the
+    "unattributed" bucket on the unit's events tab.  Once the pending
+    deployment is promoted, the new UnitAssignment's window
+    retroactively attributes them — same mechanism the metadata-
+    backfill tool uses.
+
+    Seismograph-only for v1.  SLM deployments don't follow the same
+    "field-install + verify call-home" pattern and are tracked
+    elsewhere.
+    """
+    __tablename__ = "pending_deployments"
+
+    id          = Column(String, primary_key=True, index=True)   # UUID
+    unit_id     = Column(String, nullable=False, index=True)     # FK to RosterUnit
+    captured_at = Column(DateTime, nullable=False)                # When the photo was taken
+    coordinates = Column(String, nullable=True)                   # "lat,lon" from photo EXIF
+    operator_note = Column(Text, nullable=True)                   # Free text — site memo
+
+    # Path under data/photos/{unit_id}/.  Just the filename; the unit
+    # context lives in unit_id.
+    photo_filename = Column(String, nullable=True)
+
+    # Lifecycle.
+    #   "awaiting"  — captured, not yet classified
+    #   "assigned"  — promoted to a UnitAssignment
+    #   "cancelled" — operator marked it as a mistake / abandoned
+    status      = Column(String, nullable=False, default="awaiting", index=True)
+
+    promoted_at             = Column(DateTime, nullable=True)
+    resulting_assignment_id = Column(String, nullable=True)       # FK to UnitAssignment when promoted
+
+    cancelled_at      = Column(DateTime, nullable=True)
+    cancelled_reason  = Column(Text, nullable=True)
+
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)