fix: resolve preview clip rel_path against all libraries

PreviewClipGenerator stripped a single base_path, so videos in a
non-primary library ended up with the absolute path as 'relative'.
On Windows, PathBuf::from(preview_clips_dir).join(absolute) replaces
with the absolute path, and .with_extension("mp4") on a .mp4 input
yields the input path — ffmpeg then errors out with 'cannot edit
existing files in place'.

The generator now holds Vec<Library> and strips whichever root
actually contains the video, with separator normalization to match
the rest of the code.

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

src/video/actors.rs

@@ -1,5 +1,6 @@
 use crate::database::PreviewDao;
 use crate::is_video;
+use crate::libraries::Library;
 use crate::otel::global_tracer;
 use crate::video::ffmpeg::generate_preview_clip;
 use actix::prelude::*;
@@ -500,23 +501,40 @@ pub struct GeneratePreviewClipMessage {
 pub struct PreviewClipGenerator {
     semaphore: Arc<Semaphore>,
     preview_clips_dir: String,
-    base_path: String,
+    libraries: Vec<Library>,
     preview_dao: Arc<Mutex<Box<dyn PreviewDao>>>,
 }
 
 impl PreviewClipGenerator {
     pub fn new(
         preview_clips_dir: String,
-        base_path: String,
+        libraries: Vec<Library>,
         preview_dao: Arc<Mutex<Box<dyn PreviewDao>>>,
     ) -> Self {
         PreviewClipGenerator {
             semaphore: Arc::new(Semaphore::new(2)),
             preview_clips_dir,
-            base_path,
+            libraries,
             preview_dao,
         }
     }
+
+    /// Strip whichever library root actually contains `video_path`.
+    /// Falls back to the first library if none match, so we never
+    /// accidentally emit the absolute input path as the output path
+    /// (which ffmpeg rejects as "cannot edit existing files in place").
+    fn relativize(&self, video_path: &str) -> String {
+        for lib in &self.libraries {
+            if let Some(stripped) = video_path.strip_prefix(&lib.root_path) {
+                return stripped
+                    .trim_start_matches(['/', '\\'])
+                    .replace('\\', "/");
+            }
+        }
+        video_path
+            .trim_start_matches(['/', '\\'])
+            .replace('\\', "/")
+    }
 }
 
 impl Actor for PreviewClipGenerator {
@@ -533,9 +551,10 @@ impl Handler<GeneratePreviewClipMessage> for PreviewClipGenerator {
     ) -> Self::Result {
         let semaphore = self.semaphore.clone();
         let preview_clips_dir = self.preview_clips_dir.clone();
-        let base_path = self.base_path.clone();
         let preview_dao = self.preview_dao.clone();
         let video_path = msg.video_path;
+        // Resolve against whichever library actually owns this video.
+        let relative_path = self.relativize(&video_path);
 
         Box::pin(async move {
             let permit = semaphore
@@ -543,13 +562,6 @@ impl Handler<GeneratePreviewClipMessage> for PreviewClipGenerator {
                 .await
                 .expect("Unable to acquire preview semaphore");
 
-            // Compute relative path (from BASE_PATH) for DB operations, consistent with EXIF convention
-            let relative_path = video_path
-                .strip_prefix(&base_path)
-                .unwrap_or(&video_path)
-                .trim_start_matches(['/', '\\'])
-                .to_string();
-
             // Update status to processing
             {
                 let otel_ctx = opentelemetry::Context::current();