refactor(v2): rename net_sats → principal_sats for semantic clarity

`net` is financial-accounting ambiguous (net of what?). In the
bitSpire/DCA context this column is specifically the principal the
operator distributes to LPs (gross − commission), not a generic net
amount. Renaming locally before any bitSpire firmware locks the
wire-level name; lamassu-next#44 should adopt the same name.

Scope:
- migrations.py: m003 ALTER TABLE … RENAME COLUMN, idempotent probe
  pattern matching m002. Also updates the m001 canonical schema so
  fresh installs land on the new column directly.
- models.py: `CreateDcaSettlementData.principal_sats` /
  `DcaSettlement.principal_sats`. Field-doc comment updated.
- bitspire.py: both happy path and fallback path return
  `principal_sats=…`. Reads `extra.get("principal_sats")` from the
  bitSpire payload (lamassu-next#44 should follow this rename).
- crud.py: INSERT column list + `apply_partial_dispense(
  new_principal_sats=…)` keyword.
- distribution.py: every `settlement.net_sats` → `settlement.
  principal_sats`; partial-dispense memo + helper signatures updated;
  the leg-order docblock at the top reads "principal_sats".
- tasks.py: landed-settlement log line.
- static/js/index.js: settlements-table column `principal_sats` with
  label "Principal (→ LPs)".
- templates/satmachineadmin/index.html: q-td key + binding.

All 86 unit tests still pass. No backwards-compat shim — v2-bitspire
isn't released; the rename is a clean break.

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

bitspire.py

@@ -146,9 +146,9 @@ def _parse_extra(
     super_fee_pct: float,
 ) -> CreateDcaSettlementData:
     """Happy path: bitSpire populated Payment.extra per lamassu-next#44."""
-    net_sats = _coerce_int(extra.get("net_sats"))
+    principal_sats = _coerce_int(extra.get("principal_sats"))
     fee_sats = _coerce_int(extra.get("fee_sats"))
-    if net_sats is None or fee_sats is None:
+    if principal_sats is None or fee_sats is None:
         # Missing key fields — shouldn't happen post-#44 but defensive.
         return _parse_fallback(machine, payment_hash, gross_sats, super_fee_pct)
     commission_sats = fee_sats
@@ -170,7 +170,7 @@ def _parse_extra(
         fiat_amount=fiat_amount,
         fiat_code=fiat_code,
         exchange_rate=exchange_rate,
-        net_sats=net_sats,
+        principal_sats=principal_sats,
         commission_sats=commission_sats,
         platform_fee_sats=platform_fee_sats,
         operator_fee_sats=operator_fee_sats,
@@ -193,7 +193,7 @@ def _parse_fallback(
         base_amount  = round(gross / (1 + commission_pct))
         commission   = gross - base_amount
     """
-    net_sats, commission_sats, _effective = calculate_commission(
+    principal_sats, commission_sats, _effective = calculate_commission(
         crypto_atoms=gross_sats,
         commission_percentage=machine.fallback_commission_pct,
         discount=0.0,
@@ -211,7 +211,7 @@ def _parse_fallback(
         fiat_amount=0.0,
         fiat_code=machine.fiat_code,
         exchange_rate=0.0,
-        net_sats=net_sats,
+        principal_sats=principal_sats,
         commission_sats=commission_sats,
         platform_fee_sats=platform_fee_sats,
         operator_fee_sats=operator_fee_sats,