fix(memory): make decay sweeps atomic - #7152

Summary

• commit SESSION and AGENT memory decay updates in one transaction
• count and warn about active rows whose access timestamps SQLite cannot parse
• document and test that zero TTL disables expiry for an individual scope

Verification

• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/tmp/librefang-memory-decay-target cargo test -p librefang-memory decay
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/tmp/librefang-memory-decay-target cargo clippy -p librefang-memory -p librefang-types --all-targets -- -D warnings
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/tmp/librefang-memory-decay-target cargo check --workspace --lib
• cargo fmt --all -- --check
• git diff --check

Out of scope

• automatically rewriting or deleting rows with malformed historical timestamps
• changing the configured decay schedule
• retention pruning policy changes