unify cluster db store import error handling

[fabracht]

Summary

• Follow-up to PR add partition snapshot exports for schema/index/unique/fk/constraint stores #54 review. All six cluster DB store import_* methods now propagate SerializationError on deserialize failure. Previously IndexStore::import_entries propagated via ? while DbDataStore, SchemaStore, ConstraintStore, UniqueStore, and FkValidationStore silently skipped malformed entries — a corrupt snapshot stream would either truncate or abort depending on which store hit the bad bytes first.
• Fixed IndexStore::import_entries counter accuracy. It used to increment imported even when add_entry returned Err(AlreadyExists), inflating the count on idempotent snapshot replay. Now only successful inserts are counted; AlreadyExists is treated as benign replay (so a second import of the same payload returns Ok(0)), and any other error variant propagates.
• Updated # Errors docstrings on all six methods to accurately reflect the failure modes (UTF-8 key parse failure and entity deserialize failure).
• New regression test import_entries_is_idempotent_on_replay in index_store.rs covers the counter fix.
• mqdb-cluster 0.3.2 → 0.3.3, CHANGELOG entry under 2026-05-03.

Test plan

• cargo test -p mqdb-cluster --lib — 466 passed (465 prior + 1 new test)
• cargo make clippy — clean (pedantic, all targets + wasm)
• cargo make format-check — clean
• cargo make dev — full workspace passes
• Pre-commit hook (format-check + clippy) ran on the commit and passed