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/main.py

@@ -112,6 +112,9 @@ app.include_router(admin_modules.router)
 from backend.routers import deployment_history
 app.include_router(deployment_history.router)
 
+from backend.routers import pending_deployments
+app.include_router(pending_deployments.router)
+
 # Projects system routers
 app.include_router(projects.router)
 app.include_router(project_locations.router)