docs: expand Python API reference

[stackByStack]

Description*
This PR expands docs/api.md to better document the current Python bindings and server_v2 runtime.

Changes

• Adds Python package import and installation notes.
• Documents Engine, EngineConfig, ImageRequest, and VideoRequest.
• Clarifies supported enum aliases, output types, and exception behavior.
• Expands server_v2 coverage with startup, endpoints, job lifecycle, result formats, and error schema.
• Notes that some sampler/scheduler options are accepted by the Python binding but may still be pipeline-specific at runtime.

Verification

• Cross-checked against bindings/python/src/edge_dit/* and existing Python tests.
• Documentation-only change; no code changes or test execution.