fix(api): surface session query failures - #7041

Summary

• propagate session count and list failures instead of returning an empty successful page
• return scrubbed 500 responses for session list and FTS search database failures
• keep detailed database and blocking-task errors in server logs only
• add integration coverage using deliberately removed SQLite tables

Tests

• CARGO_TARGET_DIR=/tmp/librefang-sidecar-delete/target cargo test -p librefang-api --test system_tools_sessions_test database_failure
• CARGO_TARGET_DIR=/tmp/librefang-sidecar-delete/target cargo test -p librefang-api --test system_tools_sessions_test list_sessions_honours_pagination_query
• CARGO_TARGET_DIR=/tmp/librefang-sidecar-delete/target cargo test -p librefang-api --test system_tools_sessions_test search_sessions_returns_envelope_on_no_match
• CARGO_TARGET_DIR=/tmp/librefang-sidecar-delete/target cargo clippy -p librefang-api --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Out of scope

• session mutation endpoints remain unchanged