Implement new private_api/taxa/unique_peptides endpoint

[pverscha]

Add /private_api/taxa/unique_peptides endpoint

Summary

• Adds a new GET/POST endpoint at /private_api/taxa/unique_peptides (and .json) that computes taxon-unique peptides for a given strain or species.
• Adds get_proteins_for_taxon() to the database crate: an exact term-query on taxon_id with search_after pagination to correctly handle taxa with more than 10,000 proteins.
• Adds fancy-regex dependency to support lookahead assertions in user-supplied cleavage patterns (the standard regex crate does not support lookaheads).

Endpoint contract

Request parameters

Parameter	Type	Required	Default	Description
taxon_id	u32	yes	—	NCBI taxon ID; must be at species or strain rank
cleavage_regex	string	no	[KR](?!P)	Regex matching cleavage sites; split occurs after each match (standard tryptic convention)
min_length	usize	no	5	Minimum peptide length in amino acids

Response

{
  "unique_peptides": ["AAFEDLQSLQDK", "NLFVAKNLR"],
  "total_peptides": 4821,
  "total_unique_peptides": 312
}

total_peptides is the count of deduplicated peptides after digestion and length filtering. total_unique_peptides equals unique_peptides.length.

Error cases:

• 400 — invalid cleavage_regex
• 400 — taxon_id not found in the taxon store
• 400 — taxon_id is not at species or strain rank (message includes the actual rank)

Algorithm:

1. Validate regex and taxon rank.
2. Retrieve all proteins for the taxon from OpenSearch (get_proteins_for_taxon).
3. Cleave each protein sequence at regex match boundaries (matched characters stay with the preceding fragment).
4. Filter by min_length and deduplicate.
5. Search the suffix array index for each candidate peptide (tryptic=false, equate_il=false).
6. A peptide is unique if every matching protein in the index belongs to the target taxon and the result cutoff was not reached.

[pverscha]

Merged with #101