feat(extensions): add extension loader infrastructure (#3)

## Summary

- Adds a modular extension system for Lightning.Pub enabling third-party plugins
- Provides isolated SQLite databases per extension for data safety
- Implements ExtensionContext API for accessing Lightning.Pub services (payments, Nostr, storage)
- Supports RPC method registration with automatic namespacing
- Includes HTTP route handling for protocols like LNURL
- Event routing for payment receipts and Nostr events
- Comprehensive documentation with architecture overview and working examples

## Key Components

- `src/extensions/types.ts` - Core extension interfaces
- `src/extensions/loader.ts` - Extension discovery, loading, and lifecycle management
- `src/extensions/context.ts` - Bridge between extensions and Lightning.Pub services
- `src/extensions/database.ts` - SQLite isolation with WAL mode
- `src/extensions/README.md` - Full documentation with examples

## ExtensionContext API

| Method | Description |
|--------|-------------|
| `getApplication()` | Get application info |
| `createInvoice()` | Create Lightning invoice |
| `payInvoice()` | Pay Lightning invoice |
| `getLnurlPayInfo()` | Get LNURL-pay info for a user (enables Lightning Address/zaps) |
| `sendEncryptedDM()` | Send Nostr DM (NIP-44) |
| `publishNostrEvent()` | Publish Nostr event |
| `registerMethod()` | Register RPC method |
| `onPaymentReceived()` | Subscribe to payment callbacks |
| `onNostrEvent()` | Subscribe to Nostr events |

## Test plan

- [x] Review extension loader code for correctness
- [x] Verify TypeScript compilation succeeds
- [x] Test extension discovery from `src/extensions/` directory
- [x] Test RPC method registration and routing
- [x] Test database isolation between extensions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: boufni95 <boufni95@gmail.com>
Co-authored-by: Patrick Mulligan <patjmulligan@protonmail.com>
Reviewed-on: #3

datasource.js

@@ -66,6 +66,7 @@ import { InvoiceSwaps1769529793283 } from './build/src/services/storage/migratio
 import { InvoiceSwapsFixes1769805357459 } from './build/src/services/storage/migrations/1769805357459-invoice_swaps_fixes.js'
 import { ApplicationUserTopicId1770038768784 } from './build/src/services/storage/migrations/1770038768784-application_user_topic_id.js'
 import { SwapTimestamps1771347307798 } from './build/src/services/storage/migrations/1771347307798-swap_timestamps.js'
+import { TxSwapTimestamps1771878683383 } from './build/src/services/storage/migrations/1771878683383-tx_swap_timestamps.js'
 
 
 
@@ -80,7 +81,8 @@ export default new DataSource({
         InvoiceCallbackUrls1752425992291, OldSomethingLeftover1753106599604, UserReceivingInvoiceIdx1753109184611, AppUserDevice1753285173175,
         UserAccess1759426050669, AddBlindToUserOffer1760000000000, ApplicationAvatarUrl1761000001000, AdminSettings1761683639419, TxSwap1762890527098,
         TxSwapAddress1764779178945, ClinkRequester1765497600000, TrackedProviderHeight1766504040000, SwapsServiceUrl1768413055036,
-        InvoiceSwaps1769529793283, InvoiceSwapsFixes1769805357459, ApplicationUserTopicId1770038768784, SwapTimestamps1771347307798
+        InvoiceSwaps1769529793283, InvoiceSwapsFixes1769805357459, ApplicationUserTopicId1770038768784, SwapTimestamps1771347307798,
+        TxSwapTimestamps1771878683383
     ],
 
 
@@ -89,4 +91,4 @@ export default new DataSource({
         TrackedProvider, InviteToken, DebitAccess, UserOffer, ManagementGrant, AppUserDevice, UserAccess, AdminSettings, TransactionSwap, InvoiceSwap],
     // synchronize: true,
 })
-//npx typeorm migration:generate ./src/services/storage/migrations/tx_swap_timestamps -d ./datasource.js
+//npx typeorm migration:generate ./src/services/storage/migrations/refund_swap_info -d ./datasource.js