Add SIP plan first slice (3d-5)

Introduce scheduled-investment plans as a schedule-only slice with no
cash, position or NAV side effects. Domain gains SipPolicy: amount
validation (positive, two-decimal, overflow-checked), frequency
parsing/round-trip (weekly/biweekly/monthly), nextTradeDate generating
the next due trade date off the plan anchor with month-end clamping
and weekend roll-forward, plus the isDue/executionNetAmount contract
the future executor will implement (execution itself stays
unimplemented and is marked as such; nothing debits cash in this
slice).

Persistence stores plans in sip_plans with anchor/next trade dates and
sip_plan_idempotencies under the shared replay/conflict contract; the
API exposes POST/GET /funds/{id}/sip/plans behind the bearer token.
The frontend adds a 07/SIP panel (code, amount, frequency select,
plan list with next debit date) and Web tests cover decoding,
validation, in-flight guards and stale completions. Choice note:
first debit is scheduled strictly after the anchor date (creating a
plan today never debits today).

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