Curation layer over raw_records

[lgutschow]

What

Builds the derived curation layer fresh on the immutable raw_records
layer, alongside — never replacing — the existing consolidation path. Raw stays
read-only; the curated layer only ever inserts into curated_* and is fully
rebuildable from raw.

How

Migration 008_curation.sql promotes the scaffolded curated_* tables to
load-bearing:

• curated_nodes gains scope + lineage columns (project_id, mode,
  importance, decay_class, event_time) and a scope/kind index.
• New curated_embeddings (node_id, model, embedding vector(384)) mirroring
  raw_embeddings, with an IVFFlat cosine index.
• Lineage indexes on curated_edges for reverse derived_from / supersedes
  lookups.

curation module — three passes, each scoped to a single
user/project/mode tuple and never crossing it:

• promote_working_to_episodic — every active working raw record not already
  curated becomes an episodic node plus a derived_from edge back to the raw
  id. Idempotent: the edge is the "already curated" marker.
• distill_semantic — clusters active episodic nodes by entity overlap
  (entities extracted on the fly from the source raw content, since raw carries
  no entities column), then asks the configured provider to distill each
  cluster into semantic facts, each linked by derived_from edges to every
  source raw id. Degrades to a logged no-op when the provider can't distill.
• rebuild — wipes a user's curated set and re-derives from raw across every
  project/mode bucket, proving the rebuildable-from-raw contract.

Every new node is embedded into curated_embeddings (best-effort, never fatal).

Endpoints (glass-box, behind the existing bearer auth):

• GET /records/:id/derivations — curated nodes derived from a raw id.
• POST /records/rebuild and the admin /api/curate trigger — scoped rebuild.
• A separate background scheduler task runs the rebuild on an interval, listing
  users straight from raw_records so it never touches the legacy path.

Tests

cargo test green. New coverage: promote (+ idempotency), entity-overlap
distill with all sources linked, graceful skip without distill capability,
deterministic rebuild, raw immutability under curation, per-user scope
isolation, and the derivations read. cargo fmt --all --check clean; clippy
clean on the new code; release build passes.