Refactor: Split large files + segregate IFeatureStore interfaces

[mikemcdougall]

Problem

Large files make changes hard to review and increase merge conflicts; IFeatureStore also mixes unrelated responsibilities (query, tiles, related records, edits), which drives dependency bloat and makes streaming adoption harder.

Evidence

• Large files:
  • src/Honua.Postgres/Features/FeatureStore/PostgresFeatureStore.cs (3026 LOC)
  • src/Honua.Server/Features/FeatureServer/FeatureServerEndpoints.cs (1843 LOC)
  • src/Honua.Server/Features/OgcFeatures/FeaturesEndpoints.cs (1573 LOC)
  • src/Honua.Server/Features/FeatureServer/FeatureServerHandler.cs (1571 LOC)
  • src/Honua.Server/Features/FeatureServer/Models/FeatureServerModels.cs (1362 LOC)
• Mixed responsibilities: src/Honua.Core/Features/FeatureStore/Abstractions/IFeatureStore.cs contains query, tile, related, and edit operations.

Direction

• Split large files into cohesive units (by endpoint group, query/edit, DTOs, or feature store components).
• Segregate IFeatureStore into smaller interfaces:
  • IFeatureReader / IFeatureWriter
  • IRelationshipStore
  • ITileProvider
  • IStreamingFeatureStore (already exists)
• Update DI registrations and call sites to depend on the smallest interface needed.

Tasks

• Split FeatureServerModels.cs into per‑model files.
• Split FeatureServerEndpoints.cs and FeatureServerHandler.cs into cohesive endpoint/handler units.
• Split OgcFeatures/FeaturesEndpoints.cs similarly.
• Decompose PostgresFeatureStore.cs into smaller classes (data access, query, edit, tiles, relationships).
• Introduce segregated interfaces and update implementations/usages.
• Update tests and docs/ADR as needed; ensure architecture tests still pass.

Acceptance criteria

• No files > ~800 LOC in the listed areas.
• Handler/endpoint dependencies become explicit and cohesive.
• Call sites depend on narrower interfaces rather than the full IFeatureStore.