test(manifest,gc): positive postgres-storage case + realistic gc fixture (follow-up to #144)

[hyperpolymath]

Small test-only follow-up to #144 (which closed #112 by dropping the json sidecar store).

While #112 was being resolved, two near-identical PRs were prepared concurrently (#144 and #145). #144 landed first and is now on main; #145 was a duplicate and has been closed. This PR salvages the only two bits #145 had that #144 did not — both test-only, no production-code change:

• manifest: postgres_storage_passes_storage_check — a positive companion to feat(manifest): drop unimplemented json sidecar store (V-L2-F2, #112) #144's unsupported_storage_fails. feat(manifest): drop unimplemented json sidecar store (V-L2-F2, #112) #144 covered the rejection path (json fails); this asserts the postgres dialect passes the new sidecar-storage-supported validate check, so both sides of the closed set are covered.
• gc: gc_rejects_non_sqlite_backend now uses the realistic non-sqlite value postgres instead of the now-dropped json, so the fixture reflects a backend that still exists post-V-L2-F2: implement the JSON sidecar storage backend (split from #45) #112.

The supported [sidecar].storage set stays closed to sqlite + postgres.

Testing

• cargo test — 120 lib + 21 integration/e2e pass.
• cargo fmt --check clean; cargo clippy --all-targets -- -D warnings clean.

Relates to #112 / #144.

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Findings: 80 issues detected

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🟠 High	7
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