Add funding record accumulator

[pileks]

TODO:

• Release SDK once merged
• Add migration script for funding records
• Migrate funding records

Track a time-weighted accumulator on FundingRecord that computes the integral of committed_amount over time. This captures not just how much a funder committed, but how long they've had funds committed — rewarding early and sustained commitment.

Concept

The accumulator is a discrete integral:

accumulator += committed_amount * (current_timestamp - last_update_timestamp)

Each time committed_amount changes, we "flush" the accumulated value before applying the change. The result is a running sum of committed_amount * seconds — a value that grows faster for larger and longer-held commitments.

Configurable delay

A per-launch setting accumulator_activation_delay_seconds (in seconds) defines a grace period after launch start during which the accumulator does not grow for anyone. During this window, funders can commit freely, but no time-weighted advantage accrues. Once the delay elapses, accumulation begins simultaneously for all existing commitments based on their current committed_amount.

committed_amount
      ^
      │
  500 │     ┌ ─ ─ ─┌─────────────────────┐
      │     │      |                     │ accumulation
  300 │ ┌─ ─┤ - - -|- - accumulation - - │ still active
      │ │ no acc.  |       active        │ but not recorded -
      │ │          |                     │ needs offchain calculation
      └─┴───┴──────┬─────────────────────┴────> time
      t0 t1  t2    t_act                 t3

  t0 = launch_start
  t_act = launch_start + accumulator_activation_delay_seconds

  accumulator at t3:
    funder A (300 from t1): 300 * (t3 - t_act)
    funder B (500 from t2): 500 * (t3 - t_act)

  Both start accumulating from t_act, regardless of when they funded.