refactor: decompose `src/service.ts` (3,298 lines)

[mostlydev]

Problem

src/service.ts is 3,298 lines — roughly 28% of source LOC in a single file, and it's the coordination core of the tool.

Proposal

Decompose by responsibility, e.g.:

• turn / stick state machine
• persistence (SQLite / WAL access)
• liveness / expiry / fencing
• the public service API surface

Why it matters

The coordination core is the part that most benefits from small, focused, individually-testable modules — both for review and for reasoning about the concurrency invariants (which is where the real risk in this tool lives).