Add user-facing income/revenue submission endpoint

Mirrors the existing expense submission flow so non-admin users can log
income on behalf of the organization for super-user review. New endpoint
POST /api/v1/entries/income takes invoice-key auth, creates a Beancount
transaction with the pending '!' flag, and reuses the existing
/entries/{id}/approve and /reject endpoints (which match by libra-{id}
link regardless of entry type).

Adds PermissionType.SUBMIT_INCOME granted on revenue accounts (parallel
to SUBMIT_EXPENSE on expense accounts) rather than overloading
SUBMIT_EXPENSE — the two operations target distinct account types and
should be grantable independently. Enforces AccountType.REVENUE on the
income account and AccountType.ASSET on the payment-method account;
fiat currency is required (matches the expense flow's effective
requirement). Income entries get a 'income-entry' tag and an
^inc-{entry_id} link for tracking, and surface in the existing
/entries/pending list for super-user approval.

UI work lives in the standalone webapp, out of scope here.

Closes #9

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

models.py

@@ -127,6 +127,18 @@ class RevenueEntry(BaseModel):
     currency: Optional[str] = None  # If None, amount is in satoshis. Otherwise, fiat currency code
 
 
+class IncomeEntry(BaseModel):
+    """Helper model for user-facing income/revenue submission (pending approval)"""
+
+    description: str
+    amount: Decimal  # Fiat amount in the specified currency
+    revenue_account: str  # Income/Revenue account name or ID
+    payment_method_account: str  # Asset account receiving the funds (Cash, Bank, Lightning)
+    currency: str  # Required: fiat currency code (EUR, USD, etc.)
+    reference: Optional[str] = None
+    entry_date: Optional[datetime] = None
+
+
 class LibraSettings(BaseModel):
     """Settings for the Libra extension"""
 
@@ -295,6 +307,7 @@ class PermissionType(str, Enum):
     """Types of permissions for account access"""
     READ = "read"  # Can view account and its balance
     SUBMIT_EXPENSE = "submit_expense"  # Can submit expenses to this account
+    SUBMIT_INCOME = "submit_income"  # Can submit income/revenue to this account
     MANAGE = "manage"  # Can modify account (admin level)