v0.3 step 1: migration 0004 + authors module + web form panels

Migration 0004 — authors + author_revisions + stories.author_id +
stories.author_revision_id + stories.cross_story_memory +
author_corpus. Soul versioning built in from day one per Sulkta's
locked decisions:
- authors.id immutable identity (slug + display_name + tagline + model)
- author_revisions tracks each soul revision with n monotonic
- Partial unique index 'idx_author_revisions_current' enforces
  exactly one is_current=true per author
- stories.author_revision_id pins to the exact soul used at gen
  time (so 'this was the Orson Black active when chapter 8 was
  written' is always recoverable)
- author_corpus tracks 'authored' + 'read' relationships for the
  v0.3 cross-story memory toggle

skald-core::authors module — CRUD: get_by_slug,
get_with_current_revision, get_current_revision, get_revision,
create_or_get (idempotent), add_revision (transactional, demotes
prior is_current=true), assign_to_story (also touches
author_corpus).

Web v0.1 forms (the second feedback bucket — 'no way to make new
stories', 'no options for sequels'): handlers + form panels +
POST routes for /stories/new and /stories/:id/continue. Both
create a story stub with status='seed'; actual generation will be
fired by 'skald continue' (next commit) walking seed rows.

Norse visual revamp + mobile collapse deferred — vetting full gen
is the priority per Sulkta's 'green light for v0.3'. Coming back to
the aesthetic after the pipeline works end-to-end against a real
Orson Black-authored Chapter 8 of Coast-Down.

skald-core/src/lib.rs

@@ -4,6 +4,7 @@
 //! assembly for LLM calls. The story-independence rule: nothing in
 //! this crate knows about any specific story. Every story is rows.
 
+pub mod authors;
 pub mod config;
 pub mod context;
 pub mod db;