feat: web video transcoding, admin playback, and viewer fixes

Add on-demand H.264/AAC web playback (RQ, ffmpeg) with API routes and
Next.js proxies; extend admin UI with WebPlaybackVideo and shared hooks.
Store transcode cache beside pending-photos (WEB_VIDEO_CACHE_DIR / UPLOAD_DIR)
and ignore data/web_videos. Centralize FastAPI URL helpers, optional Vite
and Next base paths for subfolder deploy, and fix modal reopen by using
router.replace when closing the home photo viewer. Include migration,
install scripts, deployment doc updates, and CI admin build env tweak.

Made-with: Cursor

viewer-frontend/scripts/install-dependencies.sh

@@ -66,6 +66,23 @@ if [ ${#MISSING_DEPS[@]} -gt 0 ]; then
                     sudo apt-get install -y ffmpeg
                 fi
             done
+        elif command -v dnf &> /dev/null; then
+            for dep in "${MISSING_DEPS[@]}"; do
+                if [ "$dep" = "ffmpeg" ]; then
+                    echo "Installing ffmpeg with dnf..."
+                    echo "Note: On EL8 (Alma/Rocky/CentOS), ffmpeg may require RPM Fusion repos."
+                    echo ""
+                    if sudo dnf -y install ffmpeg; then
+                        echo "✅ ffmpeg installed"
+                    else
+                        echo ""
+                        echo "⚠️  dnf couldn't find ffmpeg."
+                        echo "   If this is EL8, run the helper installer from the repo root:"
+                        echo "     sudo bash ../../scripts/install_ffmpeg_el8_rpmfusion.sh"
+                        echo ""
+                    fi
+                fi
+            done
         elif command -v brew &> /dev/null; then
             for dep in "${MISSING_DEPS[@]}"; do
                 if [ "$dep" = "libvips-dev" ]; then