narrate: body_md_tts column + narrate_prep pass + Kokoro routing

Two new things working together:

1. Migration 0005 adds chapters.body_md_tts (nullable). Narrate path
   prefers it over body_md when present — that's the annotated-for-
   audiobook variant. Falls back to body_md if not set.

2. New Forge::narrate_prep pass: author (or House) annotates prose
   with [breath] / [pause:Xs] / [scene] beat markers AND occasional
   humanizing narrator stumbles (em-dash repetition, self-correction,
   hesitation — sparingly, 1-3 per chapter). Apart from stumbles, the
   prose is verbatim. Author voice threads through.

3. New CLI: 'skald prepare-narration --chapter <uuid> [--author slug]
   [--overwrite]'. Records as generation_runs row kind=narrate_prep.

4. skald narrate now routes by voice.source — kokoro_* voices hit
   KOKORO_URL (Apache 2.0 stack, audiobook-tuned with the v0.2 render-
   and-stitch server), everything else hits F5_TTS_URL (voice-cloning
   path). Voice DB row carries source as the dispatch key.

Why no new tag for narrator stumbles: em-dash repetition and self-
correction are just prose patterns Kokoro reads correctly because of
its punctuation cues. No new server-side machinery.

migrations/0005_chapter_body_md_tts.sql

@@ -0,0 +1,15 @@
+-- Annotated TTS variant of each chapter's prose. NULL = not yet
+-- generated; renderers fall back to body_md when this is NULL.
+-- Otherwise this column carries the same prose with control tags
+-- inline: [pause:Xs] / [breath] / [scene] — the kokoro server
+-- interprets these into silence beats during synth.
+--
+-- Why a separate column instead of mutating body_md: the human-
+-- readable version stays clean for web inspection + future plain-
+-- text export. The TTS version is production output, regeneratable
+-- whenever the author's beat-placement taste shifts.
+ALTER TABLE chapters
+    ADD COLUMN body_md_tts text;
+
+-- generation_runs.kind already accepts any string; no check
+-- constraint to update. The new pass kind is 'narrate_prep'.