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>

tasks.py

@@ -117,7 +117,7 @@ async def _handle_payment(payment: Payment) -> None:
     logger.info(
         f"satmachineadmin: landed settlement {settlement.id} for "
         f"machine={machine.machine_npub[:12]}... "
-        f"gross={data.gross_sats}sats net={data.net_sats}sats "
+        f"gross={data.gross_sats}sats principal={data.principal_sats}sats "
         f"commission={data.commission_sats}sats "
         f"(super_fee={data.platform_fee_sats} "
         f"operator_fee={data.operator_fee_sats}){fb}"