refactor: compile the crate once — main.rs uses the library

main.rs re-declared nearly every module (`mod ai; mod database; …`)
that lib.rs also declared, so the ~100k-line crate compiled twice in
every build (lib for the src/bin tools, bin for the server) and the
test suite ran most tests twice. watcher.rs even reached into the lib
copy via `image_api::` from inside the bin copy.

The six bin-only modules (backfill, handlers, hls_stats, knowledge,
reels, watcher) move into lib.rs; main.rs keeps only startup wiring
over `use image_api::…`. A few items main calls become `pub`, and the
dead_code allowances that only existed because each copy saw half the
callers are gone. CLAUDE.md says not to reintroduce `mod` in main.rs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ai/local_llm.rs

@@ -13,9 +13,6 @@
 //! is request-scoped and stays in `ResolvedBackend`. This is only the
 //! local stack: embeddings and offline single-shot generation.
 
-// Constructed by binaries, not the server — dead code from main.rs's view.
-#![allow(dead_code)]
-
 use std::sync::Arc;
 
 use anyhow::Result;