feat: union /photos and /memories across libraries

When `library` is omitted, both endpoints now walk every configured
library root, interleave the results, and tag each row with its source
library via the parallel `photo_libraries` / per-row `library_id`
arrays. Previously the handlers fell back to the primary library,
silently hiding the rest.

Threads a parallel `file_libraries: Vec<i32>` through the sort/paginate
helpers so library attribution survives sorting and pagination.
Directory names are de-duplicated across libraries.

`get_all_with_date_taken` grows an optional library filter so memories
can scope its EXIF query per-library during the union walk.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/database/mod.rs

@@ -266,6 +266,7 @@ pub trait ExifDao: Sync + Send {
     fn get_all_with_date_taken(
         &mut self,
         context: &opentelemetry::Context,
+        library_id: Option<i32>,
     ) -> Result<Vec<(String, i64)>, DbError>;
 
     /// Batch load EXIF data for multiple file paths (single query)
@@ -523,15 +524,24 @@ impl ExifDao for SqliteExifDao {
     fn get_all_with_date_taken(
         &mut self,
         context: &opentelemetry::Context,
+        lib_id: Option<i32>,
     ) -> Result<Vec<(String, i64)>, DbError> {
         trace_db_call(context, "query", "get_all_with_date_taken", |_span| {
             use schema::image_exif::dsl::*;
 
             let mut connection = self.connection.lock().expect("Unable to get ExifDao");
 
-            image_exif
+            let query = image_exif
                 .select((rel_path, date_taken))
                 .filter(date_taken.is_not_null())
+                .into_boxed();
+
+            let query = match lib_id {
+                Some(filter_id) => query.filter(library_id.eq(filter_id)),
+                None => query,
+            };
+
+            query
                 .load::<(String, Option<i64>)>(connection.deref_mut())
                 .map(|records| {
                     records