Indexer

One indexing pass: brings a model's EmbeddingStore in line with the library. Items that were deleted (or moved into an excluded folder) are dropped. Then photos, newest first, so search covers them within minutes; then videos, whose frames are decoded on another thread into a small queue while the model works (decoding is mostly the phone's video hardware, the model the CPU, so the two overlap almost for free). Cancellation keeps everything finished so far; the next pass carries on.