fix: resolve media across libraries for video, metadata, and insights

The /video/generate and /image/metadata handlers assumed files live under
the resolved library only, which broke when a mobile client passed no
library (union mode) but the file lived in a non-primary library. Both
now fall back to scanning every configured library for an existing file.

InsightGenerator held a single base_path, so vision-model loads and
filename-date fallbacks failed for non-primary libraries. It now takes
Vec<Library> and probes each root in resolve_full_path.

/image/metadata responses now carry library_id/library_name so the
mobile viewer can surface which library a file belongs to.

Thumbnail generation at startup is now spawned on a background thread
so the HTTP server can accept traffic while large libraries backfill.

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

src/bin/populate_knowledge.rs

@@ -11,6 +11,7 @@ use image_api::database::{
     SqliteInsightDao, SqliteKnowledgeDao, SqliteLocationHistoryDao, SqliteSearchHistoryDao,
 };
 use image_api::file_types::{IMAGE_EXTENSIONS, VIDEO_EXTENSIONS};
+use image_api::libraries::{self, Library};
 use image_api::tags::{SqliteTagDao, TagDao};
 
 #[derive(Parser, Debug)]
@@ -125,6 +126,12 @@ async fn main() -> anyhow::Result<()> {
     let knowledge_dao: Arc<Mutex<Box<dyn KnowledgeDao>>> =
         Arc::new(Mutex::new(Box::new(SqliteKnowledgeDao::new())));
 
+    let populate_lib = Library {
+        id: libraries::PRIMARY_LIBRARY_ID,
+        name: "main".to_string(),
+        root_path: base_path.clone(),
+    };
+
     let generator = InsightGenerator::new(
         ollama,
         sms_client,
@@ -136,7 +143,7 @@ async fn main() -> anyhow::Result<()> {
         search_dao,
         tag_dao,
         knowledge_dao,
-        base_path.clone(),
+        vec![populate_lib],
     );
 
     println!("Knowledge Base Population");