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/files.rs

@@ -1342,8 +1342,7 @@ pub struct RealFileSystem {
 }
 
 impl RealFileSystem {
-    #[allow(dead_code)] // Used in main.rs binary and tests
-    pub(crate) fn new(base_path: String) -> RealFileSystem {
+    pub fn new(base_path: String) -> RealFileSystem {
         RealFileSystem { base_path }
     }
 }