Add liquidity tracking, budget, and platform cost history

Restore operator platform costs (Bubble, domains, Stripe) with monthly
history from March 2025 and member payments from November 2025. Track
€1,000 starting budget, cumulative burn, and remaining liquidity in the
economics dashboard. Document LQ requirements in REQUIREMENTS.md.

projects/coulomb-pricing/REQUIREMENTS.md

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+# Coulomb MVP — Liquidity & Cost Requirements
+
+## Context
+
+Coulomb Social carries **operator platform costs** (Bubble.io, domains, Stripe
+fees, future OpenRouter usage) while **customer cost-pass-through billing is not
+active yet**. Members pay a flat subscription; they are not billed for
+underlying platform spend.
+
+The Economic Observatory must make the resulting **liquidity burn** visible.
+
+## Requirements
+
+### LQ-001 — Platform cost ledger
+
+Record monthly platform costs from project start (March 2025), including fixed
+infrastructure and payment-provider variable fees when member revenue exists.
+
+### LQ-002 — Member payment ledger
+
+Record member subscription payments from first payment month (November 2025),
+separate from platform cost accrual.
+
+### LQ-003 — Budget tracking
+
+Maintain an operator liquidity budget (initial: **€1,000**) and compute
+remaining budget after cumulative platform spend minus cumulative member
+payments received.
+
+### LQ-004 — Liquidity position
+
+Report whether the project is **burning**, **neutral**, or **generating**
+liquidity each period:
+
+- `period_net = member_payments - platform_costs`
+- `cumulative_net = sum(period_net)`
+- `remaining_budget = initial_budget + cumulative_net`
+
+Negative remaining budget means the MVP has consumed more liquidity than the
+allocated budget.
+
+### LQ-005 — No customer cost billing (MVP boundary)
+
+Do not allocate platform costs to customer invoices in MVP. Cost attribution
+(OpenRouter per member, usage overage) is observatory-only until a later phase
+introduces customer-visible credits or usage billing.
+
+### LQ-006 — Historical dashboard
+
+Economics Dashboard must show:
+
+- Current-period economics (revenue, platform cost, margin)
+- Cumulative liquidity summary (budget, burn, remaining)
+- Monthly history table from March 2025
+
+## Data sources (current)
+
+| Registry | Path |
+|----------|------|
+| Budget | `data/budget.json` |
+| Platform costs | `data/costs.json` (`rate_card` + `monthly_history`) |
+| Member payments | `data/revenue.json` |
+| Membership | `data/membership.json` |
+
+Future sprints replace manual history with Bubble, Stripe, and OpenRouter imports
+while preserving the same liquidity semantics.