Add flag_state gauge metric tracking flag state changes

[kindermax]

What changed

Adds a Prometheus Gauge named flag_state with two labels — flag and project — that is set every time a flag's state changes:

• enable_flag sets the gauge to 1
• disable_flag sets the gauge to 0

Since the graph actions only receive the flag UUID, a new get_flag_metric_labels() helper in featureflags/graph/actions.py joins flag to project to resolve the human-readable names used as label values.

Why

Makes the current state of feature flags observable in Prometheus/Grafana, so dashboards and alerts can track when and which flags are toggled per project.

Notes for reviewers

• reset_flag / delete_flag intentionally do not touch the gauge: removing series is only best-effort in a per-process registry (other replicas would keep theirs anyway), so the series simply keeps its last value. This keeps the scope to explicit enable/disable transitions.
• The gauge is set after the UPDATE executes but inside the surrounding transaction, matching how the existing action metrics behave; a rollback after the action could briefly leave the gauge ahead of the database.
• Tests: test_switching_flag now asserts the gauge value matches the new flag state.
• Verified: full suite (83 passed) plus ruff/black/mypy clean on the changed files.