refactor: Extract shared base classes to eliminate duplication

[barneyjackson]

• Add _EntityBase with shared id, created_at, eq, hash for Entity and ImmutableEntity
• Add _RepositoryBase with shared session, create, ID extraction, and pagination methods
• BaseRepository and ImmutableBaseRepository become thin subclasses with only their specific behaviour
• Zero breaking changes: all public class names, signatures, and imports unchanged

[github-actions]

Name	Stmts	Miss	Cover
src/leadr/infra/webhooks/services/dependencies.py	7	7	0%
src/leadr/auth/services/nonce_tasks.py	20	7	65%
src/leadr/infra/email/__init__.py	15	5	67%
src/leadr/accounts/services/repositories.py	73	15	79%
src/leadr/registration/services/dependencies.py	25	5	80%
src/leadr/accounts/domain/account.py	42	7	83%
src/leadr/infra/cache/services/dependencies.py	7	1	86%
src/leadr/registration/services/repositories.py	117	16	86%
src/leadr/common/domain/cursor.py	71	9	87%
src/leadr/accounts/api/account_routes.py	60	7	88%
src/leadr/auth/api/identity_session_routes.py	35	4	89%
src/leadr/auth/services/repositories.py	198	22	89%
src/leadr/infra/email/adapters/repositories.py	36	4	89%
src/leadr/infra/email/adapters/smtp.py	36	4	89%
src/leadr/auth/api/device_routes.py	47	5	89%
src/leadr/scores/services/score_service.py	180	19	89%
src/leadr/games/services/repositories.py	38	4	89%
src/leadr/common/repositories.py	193	19	90%
src/leadr/accounts/api/user_routes.py	51	5	90%
src/leadr/scores/api/score_routes.py	139	13	91%
src/leadr/boards/services/board_tasks.py	78	7	91%
src/leadr/scores/api/score_event_routes.py	56	5	91%
src/leadr/auth/api/identity_routes.py	46	4	91%
src/api/routes.py	25	2	92%
src/leadr/games/api/game_routes.py	64	5	92%
src/leadr/common/utils/ip.py	13	1	92%
src/leadr/boards/api/board_state_routes.py	27	2	93%
src/leadr/boards/api/board_template_routes.py	54	4	93%
src/leadr/boards/api/run_entry_routes.py	27	2	93%
src/leadr/accounts/domain/user.py	42	3	93%
src/leadr/common/geoip.py	141	10	93%
src/leadr/auth/bootstrap.py	29	2	93%
src/leadr/boards/services/repositories.py	558	37	93%
src/leadr/boards/services/board_state_service.py	106	7	93%
src/leadr/common/domain/ids.py	93	6	94%
src/leadr/scores/api/score_schemas.py	94	6	94%
src/leadr/scores/api/score_submission_meta_routes.py	32	2	94%
src/leadr/scores/services/score_submission_meta_service.py	16	1	94%
src/leadr/boards/services/run_entry_service.py	38	2	95%
src/leadr/scores/api/score_flag_routes.py	66	3	95%
src/leadr/common/api/hooks.py	67	3	96%
src/leadr/auth/api/api_key_routes.py	47	2	96%
src/leadr/boards/domain/board.py	71	3	96%
src/leadr/boards/api/board_routes.py	123	4	97%
src/leadr/infra/webhooks/services/repository.py	31	1	97%
src/leadr/auth/services/nonce_service.py	34	1	97%
src/leadr/logging.py	69	2	97%
src/leadr/games/services/game_service.py	39	1	97%
src/leadr/registration/api/schemas.py	82	2	98%
src/leadr/registration/api/routes.py	83	2	98%
src/leadr/common/services.py	42	1	98%
src/leadr/accounts/services/account_service.py	45	1	98%
src/leadr/auth/services/api_key_service.py	100	2	98%
src/leadr/auth/services/device_service.py	52	1	98%
src/leadr/boards/services/board_service.py	68	1	99%
src/leadr/boards/domain/board_template.py	72	1	99%
src/leadr/infra/email/service.py	81	1	99%
src/leadr/registration/services/verification_service.py	90	1	99%
src/leadr/config.py	107	1	99%
src/leadr/scores/services/score_flag_service.py	173	1	99%
src/leadr/auth/dependencies.py	174	1	99%
TOTAL	7885	322	96%

137 files skipped due to complete coverage.

[github-actions]

🐳 Docker Image Built

The Docker image for this PR has been built and pushed:

ghcr.io/leadr-official/leadr-oss:fd43be7