feat(storage): add BlobReferenceSource port + registry

Step 1 of docs/plan/derived-blobs.md. Makes "who references this blob
hash" an extension point instead of SQL hardcoded in two places
(dedup_gc's reap predicate and blobs_consistency's refcount recompute,
both naming storage.files and storage.chunk_manifests directly). Adding
a blob-owning table without teaching those two risks silent orphaning:
GC sees ref_count = 0 and reaps live content.

Behaviour is unchanged — this commit only introduces the port and the
two sources that reproduce today's SQL. Wiring follows.

Two levels, not one. add_reference bumps chunk_manifests.ref_count first
and only falls back to storage.blobs.ref_count, so a reference lands on
whichever counter its hash names and the two must be recomputed
separately. RefLevel is a parameter rather than a property of a source,
because storage.files legitimately contributes at both: a manifest-less
legacy row references a chunk, a CDC row references a Blob. The
NOT EXISTS guard on the chunk-level files term is load-bearing — for a
single-chunk file the whole-file hash equals its lone chunk's hash, so
without it the row is counted at both levels.

SQL fragments rather than a per-hash count. blobs_consistency recomputes
with one query per page, the expected count inlined as correlated
subqueries; asking each source for a count per hash would turn that into
sources x rows round-trips. So sources emit a fragment the registry sums
into the existing page query, and count_references exists only for the
on-demand path where the candidate set is already filtered to
ref_count = 0.

Fragments use their own aliases (cnt_f, cnt_m) rather than the sweeps'
outer-row aliases (b, m). A fragment reusing `m` would shadow the outer
alias in the manifest sweep and silently correlate against itself;
there is a test for it.

The SQL builders are free functions so the shape can be asserted without
constructing a pool — sqlx's connect_lazy still needs a Tokio context,
and the fragments are pure string assembly anyway.

9 unit tests. fmt, clippy --all-features --all-targets, build clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/infrastructure/repositories/pg/mod.rs

@@ -1,5 +1,6 @@
 mod address_book_pg_repository;
 mod app_password_pg_repository;
+pub mod blob_reference_sources;
 mod calendar_event_pg_repository;
 mod calendar_pg_repository;
 mod contact_group_pg_repository;