fix(dashcam): range-request video streaming, modal player, async thumbnails

- app.py: replace send_file with manual Range-aware chunked generator;
  returns 206 on partial requests so browsers can seek/buffer properly.
  Thumbnail route returns 202 while ffmpeg is generating.
- app_video.py: thumbnail generation moved to background gevent greenlet
  (_generate_thumbnail_bg). get_thumbnail_path returns immediately with
  RuntimeError('generating') if not cached; ffmpeg only ever runs once
  per video (result cached in dash/thumbnails/).
- dash.html: replaced bottom card player with Bootstrap modal (modal-xl,
  centered, dark bg) — video pops up in place regardless of scroll position.
- local-videos.js: rewritten to use Bootstrap modal; thumbnail loader
  polls with fetch() and retries on 202 every 2s (up to 15x) instead of
  setting img.src directly (which can't detect 202).

Co-authored-by: Copilot <[email protected]>

modules/app_video.py

@@ -3,6 +3,7 @@ import os
 import subprocess
 import time
 
+import gevent
 from gevent import sleep
 from gevent.lock import Semaphore
 from modules import app_dash, app_sql
@@ -12,6 +13,9 @@ BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 THUMBNAIL_DIR = os.path.join(BASE_DIR, "dash", "thumbnails")
 RETENTION_SETTING = "local_video_retention_days"
 _thumbnail_lock = Semaphore()
+# Tracks which relative paths are currently being generated (set of strings)
+_generating = set()
+_generating_lock = Semaphore()
 
 
 def _local_path(relative_path):
@@ -72,24 +76,44 @@ def get_local_video_path(relative_path):
 
 
 def get_thumbnail_path(relative_path):
+    """Return the thumbnail path if it exists, or kick off background generation.
+
+    Raises:
+        FileNotFoundError: source video not found.
+        ValueError: path escapes base directory.
+        RuntimeError("generating"): thumbnail is being generated — caller should 202.
+        RuntimeError: ffmpeg failed (only propagated synchronously on first generation if
+                      the greenlet hasn't launched yet, but typically never reaches caller).
+    """
     source_path, thumbnail_path = _thumbnail_path(relative_path)
     source_mtime = os.path.getmtime(source_path)
 
-    with _thumbnail_lock:
-        if (
-            os.path.isfile(thumbnail_path)
-            and os.path.getmtime(thumbnail_path) >= source_mtime
-        ):
-            return thumbnail_path
-        if not app_dash.begin_local_video_use(relative_path):
-            raise RuntimeError(
-                "This video is currently being deleted. Try the thumbnail again shortly."
-            )
+    # Fast path: thumbnail already exists and is fresh.
+    if (
+        os.path.isfile(thumbnail_path)
+        and os.path.getmtime(thumbnail_path) >= source_mtime
+    ):
+        return thumbnail_path
 
+    # Check if generation is already running.
+    with _generating_lock:
+        if relative_path in _generating:
+            raise RuntimeError("generating")
+        _generating.add(relative_path)
+
+    # Spawn background greenlet so the HTTP thread returns immediately.
+    gevent.spawn(_generate_thumbnail_bg, relative_path, source_path, thumbnail_path)
+    raise RuntimeError("generating")
+
+
+def _generate_thumbnail_bg(relative_path, source_path, thumbnail_path):
+    """Generate thumbnail in background greenlet. Cleans up _generating on exit."""
+    try:
+        if not app_dash.begin_local_video_use(relative_path):
+            return
         try:
             os.makedirs(os.path.dirname(thumbnail_path), exist_ok=True)
             temporary_path = thumbnail_path + ".tmp.jpg"
-            error = None
             for seek_seconds in ("1", "0"):
                 command = [
                     "ffmpeg",
@@ -116,27 +140,25 @@ def get_thumbnail_path(relative_path):
                         stderr=subprocess.PIPE,
                         text=True,
                     )
-                except FileNotFoundError as e:
-                    raise RuntimeError("ffmpeg is not installed on this device.") from e
+                except FileNotFoundError:
+                    return
                 while process.poll() is None:
                     sleep(0.05)
-                _stdout, error = process.communicate()
+                process.communicate()
                 if (
                     process.returncode == 0
                     and os.path.isfile(temporary_path)
                     and os.path.getsize(temporary_path) > 0
                 ):
                     os.replace(temporary_path, thumbnail_path)
-                    return thumbnail_path
+                    return
                 if os.path.exists(temporary_path):
                     os.remove(temporary_path)
-
-            raise RuntimeError(
-                (error or "").strip()
-                or f"ffmpeg could not extract a frame from {relative_path}"
-            )
         finally:
             app_dash.end_local_video_use(relative_path)
+    finally:
+        with _generating_lock:
+            _generating.discard(relative_path)
 
 
 def delete_local_video(relative_path):