refactor: split diesel_repo into domain modules (#203)

[mariusmm]

Summary

• Convert diesel_repo.rs into a diesel_repo/ module directory with domain-specific trait impl files.
• Absorb diesel_repo_workspaces.rs into diesel_repo/workspaces.rs.
• Preserve DieselRepo and existing crate::repository::diesel_repo::* call sites (source-compatible).

Closes #203

Test plan

• cargo fmt --all
• cargo clippy -p marreq-core --all-targets --features test-helpers -- -D warnings
• cargo test -p marreq-core --features test-helpers --lib (1274 passed)

Made with Cursor

[github-actions]

Frontend SPA coverage (by module)

Module	Line Rate	Branch Rate	Function Rate	Health
api	6.9% (67 / 969)	80.0%	4.0%	❌
components	13.8% (442 / 3201)	78.9%	55.0%	❌
components/graph	48.1% (50 / 104)	100.0%	33.3%	❌
components/reports	0.0% (0 / 20)	100.0%	100.0%	❌
components/table	79.2% (38 / 48)	100.0%	100.0%	➖
context	39.7% (48 / 121)	55.0%	50.0%	❌
hooks	49.2% (98 / 199)	78.1%	81.8%	❌
lib	100.0% (57 / 57)	100.0%	100.0%	✔
pages	7.5% (510 / 6838)	77.4%	55.6%	❌
pages/catalog	0.0% (0 / 1473)	12.5%	12.5%	❌
pages/groups	0.0% (0 / 870)	0.0%	0.0%	❌
src (root)	9.4% (12 / 128)	92.9%	66.7%	❌
types	100.0% (0 / 0)	100.0%	100.0%	✔
utils	100.0% (198 / 198)	95.7%	100.0%	✔
Summary	10.7% (1520 / 14226)	81.1%	36.5%	❌

Generated from Vitest @vitest/coverage-v8 (frontend/src).

[github-actions]

Package	Line Rate	Complexity	Health
marreq-core.src	64%	0	➖
marreq-core.src.fairings	89%	0	✔
marreq-core.src.auth.guards	46%	0	❌
marreq-core.src.routes	48%	0	❌
marreq-core.src.auth.rate_limiter	94%	0	✔
marreq-cloud.src.fairings	100%	0	✔
marreq-core.src.deployment	83%	0	✔
marreq-core.src.diff	100%	0	✔
marreq-core.src.helper_functions	93%	0	✔
marreq-core.src.generators	0%	0	❌
marreq-cloud.src.services	91%	0	✔
marreq-core.src.importers	5%	0	❌
marreq-core.src.repository.cache	100%	0	✔
marreq-core.src.repository	87%	0	✔
marreq-core.src.repository.diesel_repo	7%	0	❌
marreq-core.src.models	81%	0	✔
marreq-core.src.services	90%	0	✔
marreq-core.src.services.semantic_search	75%	0	➖
marreq-core.src.validation	96%	0	✔
marreq-server.src	62%	0	➖
marreq-core.src.reqif	83%	0	✔
marreq-core.src.api	76%	0	✔
marreq-cloud.src	72%	0	➖
marreq-core.src.auth	91%	0	✔
marreq-cloud.src.api	78%	0	✔
Summary	74% (23083 / 31130)	0	➖