Feature/carrington

[mudhoney]

Add /hgc2hpc: Heliographic Carrington → Helioprojective conversion

• Adds GET /hgc2hpc (single) and POST /hgc2hpc (batch) endpoints that convert Heliographic Carrington coordinates into Helioprojective coordinates in Helioviewer's point of view, mirroring the existing /hgs2hpc routes.
• Introduces an optional observer parameter (defaulting to earth) since Carrington longitude is apparent and observer-dependent; it is validated against the solar-system bodies astropy/sunpy can resolve, returning HTTP 422 for anything else.
• Supports batch requests where a single observer and target time apply to a list of one or more coordinates, each keeping its own coord_time.
• Adds three test functions covering validation (422 cases), differential rotation, single-vs-batch consistency to 1e-9, and observer handling — full suite passes (16 tests), black and flake8 clean.
• Documents the endpoints in the README, in the auto-generated OpenAPI/Swagger docs (observer list + 422 note + multi-coordinate example), and in a standalone docs/carrington.html report.