refactor: extract convention/ domain from deposition/

[rorybyrne]

Context

Conventions (submission targets that bundle schema + hooks + file requirements) currently live in domain/deposition/. Both the deposition and ingest pathways reference conventions equally — conventions are shared infrastructure, not deposition-specific.

What

Extract ConventionService, Convention model, and convention commands/queries into their own domain/convention/ bounded context.

Files to move:

• deposition/model/convention.py → convention/model/convention.py
• deposition/service/convention.py → convention/service/convention.py
• deposition/command/create_convention.py → convention/command/create.py
• deposition/query/get_convention.py, list_conventions.py → convention/query/
• deposition/port/convention_repository.py → convention/port/repository.py
• Convention-related routes from deposition router → convention router

Why

• Ingest references conventions equally with deposition
• Having conventions in deposition/ creates a false ownership signal
• Clean bounded context with its own API, commands, queries

Effort

M — mechanical extraction, no logic changes. (human: ~1 day / CC: ~20 min)

Priority

P3 — cosmetic improvement, not blocking. Pairs well with #118 (enrichment rename).