refactor: replace irida_samples dict list with a dataclass

[georgemarselis-nvi]

Replace the irida_samples list-of-dicts with a proper dataclass (or similar typed object).

Currently irida_samples is [ { 'sample_name': str, 'project_id': int, 'r1': str, 'r2': str, 'project_name': str } ] - all string-keyed dict access, no attribute completion, no type safety.

A dataclass like:

@dataclass
class IridaSample:
    sample_name:  str
    project_id:   int
    project_name: str
    r1:           str = ""
    r2:           str = ""

would let _resolve_fastq_paths and _validate_fastq_paths write directly onto the object, eliminate the dict rebuild in the preflight loop, and give attribute access everywhere downstream.

Blocked by: #27 (initial implementation must land first).