fix(landscape): accept an empty run body; pin the builder's Alpine release

POST /landscape/runs documents an optional body, but DecodeJSON returned
io.EOF on an empty one and the handler turned that into 400 — breaking the
most common call, an all-defaults run. decodeRunRequest now treats io.EOF as
the zero request; a malformed body is still 400.

The Dockerfile's Alpine ABI coupling was enforced only by a comment: a
floating golang:1.26-alpine builder against a pinned alpine:3.24 runtime
would build green and die at startup on a libgdal soname mismatch as soon as
the Go image rolls forward. Both stages are now pinned to 3.24.

Also: the route-mount test walks the router so all six routes are asserted
(GET /slots and GET /results included), and the spec documents the 404 those
two return when migration 00011's seeded groups are absent.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

api/openapi.yaml

@@ -761,6 +761,10 @@ paths:
         Every leaf input group of the computation graph, in graph order, with
         the dataset currently filling it (any status) or `null` when nothing
         has been uploaded yet.
+
+        Yields 404 when a slot's dataset group is missing — that means
+        migration `00011_landscape.sql`, which seeds the groups, has not been
+        applied to this database.
       responses:
         "200":
           description: The input slots
@@ -769,6 +773,7 @@ paths:
               schema:
                 type: array
                 items: { $ref: "#/components/schemas/LandscapeSlot" }
+        "404": { $ref: "#/components/responses/NotFound" }
 
   /landscape/slots/{code}:
     parameters:
@@ -842,6 +847,9 @@ paths:
         Validates the params and overrides, persists the run with one pending
         node per graph node, and publishes the worker job. Returns 202: the
         graph is executed asynchronously, poll `GET /landscape/runs/{id}`.
+
+        Every field has a default, so the body is optional: sending no body at
+        all (or `{}`) starts a default incremental run.
       requestBody:
         required: false
         content:
@@ -879,6 +887,10 @@ paths:
         One entry per output group of the graph with its freshest `ready`
         raster, the COG URL to render it, and the group's SLD URL when one has
         been uploaded. `latest` is `null` while nothing has been computed.
+
+        Yields 404 when an output group is missing — that means migration
+        `00011_landscape.sql`, which seeds the groups, has not been applied to
+        this database.
       responses:
         "200":
           description: The result layers
@@ -887,6 +899,7 @@ paths:
               schema:
                 type: array
                 items: { $ref: "#/components/schemas/LandscapeResult" }
+        "404": { $ref: "#/components/responses/NotFound" }
 
 components:
   parameters: