Masterdata: GET /api/v1/masterdata/series list + get-by-id

[DutchJaFO]

Background

Depends on #183 (shared list-endpoint infrastructure).

SeriesEntity (src/Quotinator.Engine/Entities/Series.cs), added by #179, has no repository or
endpoint of any kind today. This issue gives Series its first read access, needed before #180's
overlay-file work can be verified against real API responses and before duplicate-Series discovery
becomes possible.

What needs to be done

1. GET /api/v1/masterdata/series — paginated list, using List-endpoint shared infrastructure for the masterdata and Conversations list endpoints #183's IListableRepository<SeriesEntity>
   (Series' first-ever repository registration) + PageResponse<T> + shared pagination-clamp helper.
   Response items include Id, Name, UniverseId (nullable), CompletenessStatus.
2. GET /api/v1/masterdata/series/{id} — single Series by id, using List-endpoint shared infrastructure for the masterdata and Conversations list endpoints #183's shared not-found helper.
   {id} matches case-insensitively.
3. Both endpoints use RateLimitPolicies.Api, tag ApiTags.MasterData, and [Description] attributes.
4. No entity-specific filters yet (e.g. ?universeId=) — deferred per List-endpoint shared infrastructure for the masterdata and Conversations list endpoints #183's filter convention.
5. Update README.md and addon/DOCS.md's endpoint tables.

Expected tests

Test class	Test method	Starts
New: Quotinator.Api.Tests	GetAllSeries_ReturnsPaginatedResults	❌
New: Quotinator.Api.Tests	GetSeriesById_ExistingId_ReturnsSeries	❌
New: Quotinator.Api.Tests	GetSeriesById_UnknownId_Returns404	❌
New: Quotinator.Api.Tests	GetSeriesById_LowercaseId_MatchesCaseInsensitively	❌

Definition of done

• All expected tests listed above start red before implementation
• All requirements implemented
• All expected tests pass (green)
• No regression in related tests
• Findings summarised in a closing comment

[DutchJaFO]

Released in v1.8.0.

#	Status	Requirement	Method	Verification
1-6	✅	Get-by-id with 404/case-insensitive/malformed-id handling; completenessStatus serializes cleanly	Unit test	SeriesEndpointsTests.GetSeriesById_*
7-14	✅	Full pagination matrix; OpenAPI registration; tag/rate-limit live	Unit test	GetAllSeries_*
17-20	✅	Universe reference resolves to {id, name}; null for no-Universe/soft-deleted-Universe; batched resolution	Unit test	GetSeriesById_SeriesHasUniverse_*, _SeriesHasNoUniverse_*, _UniverseSoftDeleted_*, GetAllSeries_MultipleSeriesWithUniverse_*
21-22	✅	Docs updated; no regression	Doc + Unit test	1440 tests, 0 warnings, 0 errors
23-24	✅	T1/T2 — clean startup, live Universe reference resolution confirmed	Live	Confirmed 2026-07-19 (combined pass across #184–#189/#204/#205)