Consolidate hardcoded default vessel specifications

[SL-Mar]

Problem

Default vessel specs (49,000 DWT MR tanker) are hardcoded in multiple places:

• api/main.py line ~368-378: VesselConfig Pydantic model with defaults
• api/main.py line ~426: VesselSpecs() uses module-level defaults
• tests/integration/test_api.py line ~87-96: test fixtures repeat the same values

Impact

• Changing default specs requires updates in 3+ places
• Risk of inconsistency between code paths
• Tests may not reflect actual defaults

Steps to fix

1. Create a single canonical source (e.g. DEFAULT_VESSEL constant or config)
2. Reference it from VesselConfig, global state init, and test fixtures
3. Document the default vessel in one place

Files

• api/main.py
• tests/integration/test_api.py