docs: add BACKLOG.md for near-term open tasks

Track loose ends between milestones: the pgvector spike's final coverage
snapshot (pending CLIP re-run completion), the M1 validation-gate run on a hard
sample, and the M1.5 least-privilege read-only role. Linked from ROADMAP.

docs/BACKLOG.md

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+# Backlog
+
+Small, near-term open tasks that don't warrant a full milestone. Milestones live in
+`docs/ROADMAP.md`; this file tracks the loose ends between them. Remove an item when it's done.
+
+## Open
+
+- [ ] **pgvector spike — final coverage snapshot.** The feasibility spike has *passed* (embeddings
+  readable, `assetId → asset.id` join clean, shape pinned: `smart_search.embedding`, 1152-dim,
+  cosine `<=>`). The CLIP re-run with the stronger model is still in progress (coverage ~70% and
+  climbing as of 2026-06-27). When it reaches ~100%, run `scripts/pgvector_spike.py` once (no
+  flags) and commit the refreshed findings doc to record final coverage.
+  See the "Definition of done" in `docs/superpowers/specs/2026-06-27-pgvector-embedding-spike-design.md`.
+
+- [ ] **M1 validation gate — run on a hard, GPS-poor sample.** M1 shipped, but the acceptance-bar
+  run on a hard sample (not the easy last trip) is still pending before widening.
+  See `docs/M1-validation-gate.md` and the M1 row in `docs/ROADMAP.md`.
+
+## Later / dependent
+
+- [ ] **M1.5 — provision a least-privilege read-only Postgres role.** When M1.5 builds the pgvector
+  reader, replace the spike's session-level read-only guard (using Immich's write-capable `postgres`
+  user) with a dedicated `SELECT`-only role. Recipe is in the findings doc. Also: the pgvector reader
+  belongs in `shared/photoflow/immich` (the only Immich client) and any SQLite access in
+  `shared/photoflow/core` (the only SQLite owner) — not in app-level code.