feat(places): backend for the photo map (geo clusters API)

Phase 1 server side, gated on OXICLOUD_ENABLE_PLACES (off by default):

- migration: partial index on storage.file_metadata(longitude, latitude).
- FileBlobReadRepository::list_geo_clusters — plain-SQL grid aggregation
  (no PostGIS) scoped to the caller's own non-trashed photos, returning a
  centroid, count and a representative file id per non-empty cell.
- PlacesService (caller_id-scoped; user-scoped data needs no authz check,
  mirroring RecentService) with a zoom→cell-size mapping.
- GET /api/photos/geo?bbox=w,s,e,n&zoom=N returning GeoCluster[]. The route
  is mounted only when the Places service is present, and is registered in
  the OpenAPI path list.

The map frontend (PMTiles serving + MapLibre module) is deferred pending
the basemap-sourcing and vendoring decision.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JW6ghFMDtnRYuYNzZhb47M

src/application/dtos/geo_dto.rs

@@ -0,0 +1,26 @@
+//! DTOs for the "Places" (photo map) feature.
+
+use serde::Serialize;
+use utoipa::ToSchema;
+
+/// A geographic bounding box in decimal degrees.
+#[derive(Debug, Clone, Copy)]
+pub struct GeoBounds {
+    pub west: f64,
+    pub south: f64,
+    pub east: f64,
+    pub north: f64,
+}
+
+/// A clustered group of geotagged photos within one aggregation cell.
+#[derive(Debug, Clone, Serialize, ToSchema)]
+pub struct GeoCluster {
+    /// Cluster centroid longitude.
+    pub lng: f64,
+    /// Cluster centroid latitude.
+    pub lat: f64,
+    /// Number of photos in the cluster.
+    pub count: i64,
+    /// A representative photo id, for the cluster thumbnail.
+    pub sample_file_id: String,
+}

src/application/dtos/mod.rs

@@ -10,6 +10,7 @@ pub mod favorites_dto;
 pub mod file_dto;
 pub mod folder_dto;
 pub mod folder_listing_dto;
+pub mod geo_dto;
 pub mod grant_dto;
 pub mod i18n_dto;
 pub mod pagination;