Extract Collector Store seam

[Amorpheuz]

Summary

• introduce a small Store module that owns SQLite connections, write serialization, canonical Event inserts, and Event row conversion
• move shared timestamp behavior into timeutil and rewire Collector readers and writers through the new seams
• consolidate database test setup, enforce the private-member boundary with Ruff, and add reproducible local/CI checks
• pin GitHub Actions to immutable commit SHAs

Why

Collector persistence behavior was spread across db.py, analytical modules, and tests. That made transaction ownership unclear, allowed private database helpers to leak across modules, and left local and CI validation inconsistent.

This change implements the Store boundary described by ADR-0001 while preserving the existing schema and response behavior.

Impact

Contributors get one explicit persistence boundary: analytical readers retain SQL ownership, while mutations go through serialized Store write transactions. Runtime API, Session, Event, Origin, ingest, and Insights behavior remain unchanged.

Validation

• 135 passed on Python 3.12
• ruff check backend
• Store structural acceptance checks
• parallel standards and specification review with no blocking findings
• workflow YAML parse check

Follow-up

• Mark collector-checks as required in the default branch ruleset after this workflow first runs.