refactor: extract fee constants and consolidate dependency checks

[Folex1275]

Refactor: Code deduplication and maintenance improvements

Closes #960
Closes #961
Closes #962
Closes #963

This PR addresses four maintenance issues identified in code review.

Changes

1. Deduplicate dependent-route scan loop in router-core

• Extracted common "route has dependents" check into route_has_dependents() helper
• Eliminates copy-paste logic between remove_route() and remove_route_internal()
• Future changes to dependency checking now only need updating in one location

2. Extract duplicated fee constants in rpc.rs

• Introduced module-level constants: BASE_FEE, SURGE_THRESHOLD_BPS, SURGE_MULTIPLIER, NORMAL_MULTIPLIER
• Replaced hardcoded values in both simulate() and heuristic_estimate()
• Ensures consistency between RPC path and heuristic fallback path

3. Document unused fee_bps field

• Added doc comment to SimulateRequest.fee_bps noting it's currently unused/reserved
• Clarifies that fee estimation uses hardcoded constants for now
• Prevents API consumer confusion about expected behavior

4. Normalize CI workflow formatting

• Changed branches: [ main, develop ] to branches: [main, develop] in metrics-exporter.yml
• Matches bracket-spacing convention used in all other workflow files
• Improves consistency across .github/workflows/

Impact

• No behavioral changes
• Improves maintainability and code clarity
• Reduces risk of drift in duplicated logic

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