fix(docs): show the no-data part of Sentinel-2 previews as transparent

STAC Browser draws an item's `preview` COG, the asset with the
`overview` role. Earth Search's preview declares no no-data value, so the
empty part of a partially covered tile came out black (97% of it on
S2B_T39QWD_20250413T071904_L2A).

- Notebook 02 marks 0 as the preview's no-data value before loading the
  items. ol-stac passes it to OpenLayers, which draws those pixels
  transparent.
- STAC Browser gets SB_displayOverviewsForChildren=true in the chart and
  in both compose files. Collection and search pages, such as the one
  notebook 02 embeds, then draw the preview COG instead of the JPEG
  thumbnail, which can't be transparent.

Verified against stac-browser 5.1.0 in Chrome and Firefox: the black
share of the no-data area dropped from 0.97 to 0. The field survives
pypgstac upsert, stac-fastapi and stac-auth-proxy unchanged.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: lhoupert

docker-compose.yml

@@ -195,6 +195,8 @@ services:
       - 8080:8080
     environment:
       SB_catalogUrl: "http://localhost:8084"
+      # draw items' preview COGs (no-data transparent) on collection pages, not JPEG thumbnails
+      SB_displayOverviewsForChildren: "true"
       # # Chapter 8: log in against the mock OIDC server. `historyMode` must be
       # # "history" for OIDC (it is STAC Browser's default), and the redirect URI is
       # # always <browser origin>/auth -- here http://localhost:8080/auth.

docs/02-database.ipynb

@@ -243,7 +243,8 @@
    "id": "34af967a-b837-4910-afde-6145582cf216",
    "metadata": {},
    "source": [
-    "You are going to upload these items to the items table in the `pgstac` database but to do so you need to make sure the items' collection ID matches an existing collection - set it to match the collection that you uploaded in the previous step."
+    "You are going to upload these items to the items table in the `pgstac` database but to do so you need to make sure the items' collection ID matches an existing collection - set it to match the collection that you uploaded in the previous step.\n",
+    "\nEarth Search's `preview` image of each item declares no no-data value, so STAC Browser would draw the empty part of a partially covered tile in black. Marking `0` as its no-data value makes that part transparent."
    ]
   },
   {
@@ -256,6 +257,10 @@
     "# override the collection id to match your new collection\n",
     "for item in items:\n",
     "    item.set_collection(my_collection)\n",
+    "    # the `preview` image declares no no-data value: mark 0 so STAC Browser\n",
+    "    # draws the empty part of the tile transparent instead of black\n",
+    "    if \"preview\" in item.assets:\n",
+    "        item.assets[\"preview\"].extra_fields[\"nodata\"] = 0\n",
     "\n",
     "items[0]"
    ]

participant/chart/values.yaml

@@ -194,6 +194,8 @@ containers:
     env:
       SB_pathPrefix: /browser/
       SB_catalogUrl: "{origin}/stac/"
+      # draw items' preview COGs (no-data transparent) on collection pages, not JPEG thumbnails
+      SB_displayOverviewsForChildren: "true"
       SB_authConfig: >-
         {"type":"openIdConnect",
         "openIdConnectUrl":"{origin}/oidc/.well-known/openid-configuration",

participant/compose.participant.yml

@@ -192,6 +192,8 @@ services:
     environment:
       SB_pathPrefix: /browser/
       SB_catalogUrl: http://localhost:18888/stac/
+      # draw items' preview COGs (no-data transparent) on collection pages, not JPEG thumbnails
+      SB_displayOverviewsForChildren: "true"
       # Chapter 8 login against this stack's mock-oidc. The redirect URI is
       # <origin><pathPrefix>auth = http://localhost:18888/browser/auth.
       SB_authConfig: >-