spp_api_v2: If-Match is optional on PUT/PATCH although api-design.md requires it #561

Problem

docs/principles/api-design.md says PUT and PATCH require If-Match for optimistic locking. Every v2 write
endpoint treats it as optional (if if_match: ...), so a client that omits it can silently overwrite a concurrent
change:

• spp_api_v2/routers/individual.py (PUT ~395, PATCH ~470)
• spp_api_v2/routers/group.py (PUT ~368, PATCH ~444)
• spp_api_v2_change_request/routers/change_request.py (~369)
• spp_api_v2_programs/routers/program_membership.py (PUT)

Proposal

Decide between (a) enforcing the principle (428 Precondition Required when If-Match is missing — a breaking change
for existing clients, e.g. the OpenFn adaptor does not send it by default), or (b) amending the principle to "optional,
checked when present". Either way, add a shared "current version of a record" helper so the microsecond versionId
format cannot drift again (the ProgramMembership PUT drifted to float seconds and always 409'd, fixed in PR #555).

Context

Noted during #554 / PR #555.