docs: accounts payable module page

[futurebuildai]

docs: accounts payable module page

Adds docs/modules/ap.md for the AP module: the vendor bill (ApInvoice), the approve and void transitions, the vendor payment (ApPayment), the aging by vendor and the GL postings (2010 Accounts Payable, 1010 Cash). Lists the seven AP routes under /api/v1/ap, the bill and line fields, the lifecycle with its blockers (has_payments, entry_not_linked, entry_unbalanced, period_closed, account_missing, line_needs_account, invoice_not_approved, exceeds_open), the events the module writes (vendor_invoice.created, .approved, .partial, .paid, .voided), the admin/owner/finance role guard behind the branch wall and the stated limits from C4-1b (unlinked legacy bills stay approved and payable; across-branch payments show whole). Adds the page to docs/modules/README.md's module table and the ap segment to the convention block, and removes AP from the not-in-brief sentence.

Every sentence on the page traces to a row in the claims table below.

Claims table (rows A through M, source paths and the proof that backs each row): see /home/colton/Desktop/FBHQ/gable-v1/mm/docs-ap-claims.md (off repository).

The groups:

• A. Routes: 7 AP routes; census rows in core/api/ROUTES.txt.
• B. Bill fields: every property of ApVendorInvoice, ApVendorInvoiceSummary and ApVendorInvoiceLine.
• C. Payment fields: ApPayment and the ApCreatePaymentRequest body.
• D. Aging: ApAgingSummary bucket fields.
• E. Lifecycle: statuses, edges in core/internal/ap/model.go:53-62, blockers has_payments, entry_not_linked, entry_unbalanced, invoice_not_approved, exceeds_open, plus the If-Match 428 / stale_revision 409 rule.
• F. Posting failure blockers: period_closed, account_missing, entry_unbalanced, line_needs_account mapped in service.go.
• G. Scopes and roles: scoped("admin", "owner", "finance") at serve.go:679; BranchGuard.CheckPayloadBranch for the payload branch_id.
• H. Events the module writes: vendor_invoice.created, .approved, .partial, .paid, .voided from service.go:49-55; written in transaction per ADR 0003 section 2.
• I. GL postings: approve builds entry from lines (pro rata tax, last line takes remainder, credit to 2010); pay entry is DR 2010 / CR 1010; void reverses through gl.PostReversal. Accounts AccountCodeAP = "2010" and AccountCodeCash = "1010".
• J. Branch wall behaviour: list, get, transitions, payments list and aging all hold the wall; a payment spanning branches shows whole.
• K. Stated limits: unlinked legacy bills stay approved and payable (no route links an entry to a bill today); bills with the removed sync's one leg unbalanced entry likewise.
• L. Money ceilings: tax_cents, lines[i].unit_price_ten_thousandths, lines[i].quantity, lines subtotal, lines-and-tax together, amount_cents: all 999999999999 cents; unit_price 999999999999 ten-thousandths. Defined at input.go:261-265.
• M. ADR rule sentences: ADR 0008 sections 7.4, 12 and 13; ADR 0001 sections 4, 6, 7, 7a, 9, 11 and 12; ADR 0007 section 5.5; ADR 0003 sections 1, 2, 3 and 5.

File changes:

• docs/modules/ap.md: new page.
• docs/modules/README.md: adds the Accounts payable row and the ap segment in the convention block; removes AP from the "not in the brief's list" sentence.

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