Add configurable label normalization and event filtering

[darthfork]

Summary

• Closes Add configurable label normalization and event filtering #40 by adding an opt-in label policy so operators can filter repositories, branches, and workflows and optionally collapse branch labels into default / release / feature.
• Filtered webhooks are still acknowledged and deduplicated; they skip business metrics/run state and increment promgithub_event_filtered_total.
• Default scrape shape is unchanged. Helm values and usage docs cover the recommended production starting point.

Test plan

• go test ./src and go test -tags=integration ./src
• Start the service with no label-policy env vars and confirm workflow metrics still use raw branch="main"
• Set PROMGITHUB_REPO_DENYLIST for a fixture repo and confirm /webhook returns 202 while promgithub_event_filtered_total{reason="repository"} increments
• Set PROMGITHUB_NORMALIZE_BRANCHES=true and confirm main is exported as branch="default"
• Set an invalid PROMGITHUB_BRANCH_ALLOW_REGEX and confirm the process fails at startup

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