Server-only helper functions (get_current_admin_user, row_to_post_list,
row_to_post_full, sync_tags, clean_tags) are defined behind
#[cfg(feature = "server")] in helpers.rs, but their imports in consumer
files were not gated, causing WASM build failures.
- Move posts.rs (891 lines) to posts/ with 11 submodules
- Extract shared tag sync logic into helpers::sync_tags
- Extract tag cleaning into helpers::clean_tags
- All external import paths unchanged via mod.rs re-exports
- Zero behavioral changes, pure structural refactor
