The frontend was wired against /api/documents/{id}/chunks/* (canonical
doc-centric chunkset) but the backend never exposed those routes — the
chunk tab in the doc workspace 404'd. The domain entities (Chunk,
ChunkEdit, ChunkPush) and persistence repos already existed since #205;
what was missing was the service + API layer that connects them.
ChunkService owns all canonical chunkset invariants (sequence ordering,
soft-delete + audit log atomicity) and shares the chunker port with
AnalysisService so chunking strategy stays a single implementation.
AnalysisService grew a duck-typed promoter hook that copies the chunks
of the first successful analysis into the canonical chunkset. The hook
is idempotent so subsequent ad-hoc analyses (Studio / OCR Debug) never
overwrite hand-edited state.
Routes added (all additive, /api/documents prefix):
GET /{id}/chunks
POST /{id}/chunks
PATCH /{id}/chunks/{chunkId}
DELETE /{id}/chunks/{chunkId}
POST /{id}/chunks/{chunkId}/split
POST /{id}/chunks/merge
POST /{id}/rechunk
GET /{id}/tree
GET /{id}/diff?store=...
POST /{id}/chunks/push
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