fix: seed import update mode fills gaps only — MSFS is primary

Changes  behavior: ALL fields now use ONLY_IF_EMPTY
policy for existing assets. Previously, AUTO fields (name, make, model,
company, place, building_name, floor, etc.) always overwrote existing
data, clobbering MSFS-synced and manually-cleaned records.

- db_writer.py: update() treats every field as 'skip if existing
  value non-null' — same as old PRESERVE behavior but universal
- sync.py: UPDATE path now fetches current values and only sets
  fields that are currently NULL/empty
- New assets (not found by machine_id) still insert with full seed data
- sync.py docstring and stats output updated to reflect new policy

Closes #13 — seed import no longer overwrites MSFS data

db_writer.py

@@ -350,10 +350,12 @@ class DatabaseWriter:
         """
         Update mode: Upsert by machine_id.
         
-        PRESERVE fields (lat, lng, photo_path, install_date, deployed, pulled_date):
+        MSFS is primary data source — seed import fills gaps only.
+        
+        ALL fields (except machine_id PK) are treated as ONLY_IF_EMPTY:
             Skip update if existing value is non-null.
-        AUTO fields: Always overwrite.
-        machine_id: Never changes (used as match key).
+        
+        New assets (not found by machine_id) are inserted with full data.
         
         Returns dict with counts.
         """
@@ -408,27 +410,21 @@ class DatabaseWriter:
                     stats["errors"] += 1
                     print(f"  ⚠️  Error inserting machine {mid}: {e}")
             else:
-                # UPDATE existing record with preserve/auto rules
+                # UPDATE existing record — MSFS is primary, seed fills gaps
                 existing_dict = dict(existing)
                 updates = {}
-                preserved_count = 0
+                skipped_count = 0
 
                 for col in update_cols:
+                    if col == "machine_id":
+                        continue  # PK — never update
                     new_val = row.get(col)
                     old_val = existing_dict.get(col)
-
-                    if col in PRESERVE_FIELDS:
-                        # PRESERVE: skip if existing value is non-null
-                        if old_val is not None and old_val != "":
-                            preserved_count += 1
-                            continue  # Don't update
-                        # If old is NULL, we can set it
-                        updates[col] = new_val
-                    elif col == "machine_id":
-                        continue  # PK — never update
-                    else:
-                        # AUTO: always overwrite
-                        updates[col] = new_val
+                    # Skip if existing value is non-null/empty (MSFS or user data wins)
+                    if old_val is not None and old_val != "":
+                        skipped_count += 1
+                        continue
+                    updates[col] = new_val
 
                 if updates:
                     # Build dynamic UPDATE
@@ -448,7 +444,7 @@ class DatabaseWriter:
                             set_values,
                         )
                         stats["updated"] += 1
-                        stats["preserved"] += preserved_count
+                        stats["preserved"] += skipped_count
                     except Exception as e:
                         stats["errors"] += 1
                         print(f"  ⚠️  Error updating machine {mid}: {e}")
@@ -497,7 +493,7 @@ def write_update(machines, db_path):
         final_count = writer.get_machine_count()
         print(f"  ✅ Inserted: {stats['inserted']}")
         print(f"  🔄 Updated: {stats['updated']}")
-        print(f"  🛡️  Preserved fields: {stats['preserved']}")
+        print(f"  🛡️  Existing data preserved: {stats['preserved']} fields")
         print(f"  ⏭️  Skipped (no changes): {stats['skipped']}")
         print(f"  📊 Total in DB: {final_count}")
         if stats["errors"]: