Engine: per-query statement prepares, non-transactional paired writes, N+1 search lookups, crash traps #45

Findings from the repo review of Sources/DigUpKit / Sources/LlamaRuntime:

• Database.swift re-prepares every SQL statement on every call (no cache) — hot paths pay sqlite3_prepare_v2 per insert/update/count; transaction(_:) is not reentrant
• IndexStore.swift: move, requeue, insert run their paired statements outside any transaction — a mid-way failure leaves the files table and the keyword index out of step; failed embeddings store a 0-byte blob instead of NULL (phantom vectors); pending() interpolates an unbounded IN (…) list
• Searcher.swift does one files and one segments SELECT per hit (N+1)
• MediaExtractors.swift force-unwrapped baseAddress on zero-length sample buffers (trap); ImageSizing.targetSize divides by height * width and traps on zero
• LlamaEmbedder hardcodes the 30 s audio window instead of the configured audioWindow

Fixed by #38.