graph-population PR B: route wiring + backfill endpoint

[amitpaz1]

Summary

Wires PR A's extraction service into the create-time path and adds the HTTP backfill endpoint. After this PR + reinstall, every new memory / observation triggers graph extraction as a fire-and-forget task; the 50+ pre-existing unenriched memories recover via `POST /v1/graph/backfill` (or `lore graph-backfill` CLI).

Wiring

• `routes/memories.py` — after the existing `enrich_memory_async` task, fire `graph_svc.extract_and_persist` if `graph_svc.is_enabled()`. Auto-on iff `claude` is on PATH; explicit override via `LORE_GRAPH_EXTRACTION_ENABLED`.
• `routes/observations.py` — same hook. Closes the silent gap from the audit: observations were the bulk-write tier the dream subagent saves through, so without this the graph stayed empty even with PR A landed.

New route — `POST /v1/graph/backfill`

• Body: `{limit?: 1-100, force?: bool, project?: str}`.
• Default: walks `list_memories_without_mentions` and runs extraction.
• `force=true`: pulls from `list_memories`, re-extracts memories that already have mentions (use after prompt/model revision).
• Returns `{processed, failed, results, enabled}`. `enabled=false` when extraction is disabled so callers don't silently see `processed=0` and assume success.
• Auth: writer/admin.

CLI — `lore graph-backfill`

• Replaces the dead SDK-side path (which returned 0 silently in HTTP-store mode).
• POSTs to `/v1/graph/backfill` via `HttpStore._request`, drains pages until `processed+failed == 0` (with a 50-page safety cap).
• Prints clear "extraction disabled" message when the server reports `enabled=false` instead of exiting 0 silently.

Test plan

• Manual end-to-end (PR A install): ran extraction on 8 unenriched memories live → 12 entities + 28 entity_mentions + 13 relationships materialized; cross-memory dedup works (`PR graph-population PR A: extraction service + store ops #50` referenced from 5+ memories, single entity row).
• `ruff check src/ tests/` — clean
• 12 new tests in `tests/test_graph_extraction_wiring.py`: create-time fire-and-forget on both routes (enabled + disabled), backfill happy path, no-unenriched edge case, `force=true` switch to `list_memories`, extraction-error → failed count, CLI body shape, CLI page draining, CLI disabled-message branch.
• Existing `tests/test_enrichment_memories.py` fixture now sets `LORE_GRAPH_EXTRACTION_ENABLED=false` so its existing `assert_called_once` counts stay deterministic regardless of `claude` on PATH.
• `pytest tests/ --ignore=test_http_store_integration` — 2815 passed (2803 baseline + 12 new), 0 failed.
• CI green

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