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

proto/lnd/walletunlocker.client.ts

@@ -1,4 +1,4 @@
-// @generated by protobuf-ts 2.8.1
+// @generated by protobuf-ts 2.11.1
 // @generated from protobuf file "walletunlocker.proto" (package "lnrpc", syntax proto3)
 // tslint:disable
 import type { RpcTransport } from "@protobuf-ts/runtime-rpc";
@@ -50,7 +50,7 @@ export interface IWalletUnlockerClient {
      * method should be used to commit the newly generated seed, and create the
      * wallet.
      *
-     * @generated from protobuf rpc: GenSeed(lnrpc.GenSeedRequest) returns (lnrpc.GenSeedResponse);
+     * @generated from protobuf rpc: GenSeed
      */
     genSeed(input: GenSeedRequest, options?: RpcOptions): UnaryCall<GenSeedRequest, GenSeedResponse>;
     /**
@@ -68,7 +68,7 @@ export interface IWalletUnlockerClient {
      * seed, then present it to the user. Once it has been verified by the user,
      * the seed can be fed into this RPC in order to commit the new wallet.
      *
-     * @generated from protobuf rpc: InitWallet(lnrpc.InitWalletRequest) returns (lnrpc.InitWalletResponse);
+     * @generated from protobuf rpc: InitWallet
      */
     initWallet(input: InitWalletRequest, options?: RpcOptions): UnaryCall<InitWalletRequest, InitWalletResponse>;
     /**
@@ -76,7 +76,7 @@ export interface IWalletUnlockerClient {
      * UnlockWallet is used at startup of lnd to provide a password to unlock
      * the wallet database.
      *
-     * @generated from protobuf rpc: UnlockWallet(lnrpc.UnlockWalletRequest) returns (lnrpc.UnlockWalletResponse);
+     * @generated from protobuf rpc: UnlockWallet
      */
     unlockWallet(input: UnlockWalletRequest, options?: RpcOptions): UnaryCall<UnlockWalletRequest, UnlockWalletResponse>;
     /**
@@ -84,7 +84,7 @@ export interface IWalletUnlockerClient {
      * ChangePassword changes the password of the encrypted wallet. This will
      * automatically unlock the wallet database if successful.
      *
-     * @generated from protobuf rpc: ChangePassword(lnrpc.ChangePasswordRequest) returns (lnrpc.ChangePasswordResponse);
+     * @generated from protobuf rpc: ChangePassword
      */
     changePassword(input: ChangePasswordRequest, options?: RpcOptions): UnaryCall<ChangePasswordRequest, ChangePasswordResponse>;
 }
@@ -128,7 +128,7 @@ export class WalletUnlockerClient implements IWalletUnlockerClient, ServiceInfo
      * method should be used to commit the newly generated seed, and create the
      * wallet.
      *
-     * @generated from protobuf rpc: GenSeed(lnrpc.GenSeedRequest) returns (lnrpc.GenSeedResponse);
+     * @generated from protobuf rpc: GenSeed
      */
     genSeed(input: GenSeedRequest, options?: RpcOptions): UnaryCall<GenSeedRequest, GenSeedResponse> {
         const method = this.methods[0], opt = this._transport.mergeOptions(options);
@@ -149,7 +149,7 @@ export class WalletUnlockerClient implements IWalletUnlockerClient, ServiceInfo
      * seed, then present it to the user. Once it has been verified by the user,
      * the seed can be fed into this RPC in order to commit the new wallet.
      *
-     * @generated from protobuf rpc: InitWallet(lnrpc.InitWalletRequest) returns (lnrpc.InitWalletResponse);
+     * @generated from protobuf rpc: InitWallet
      */
     initWallet(input: InitWalletRequest, options?: RpcOptions): UnaryCall<InitWalletRequest, InitWalletResponse> {
         const method = this.methods[1], opt = this._transport.mergeOptions(options);
@@ -160,7 +160,7 @@ export class WalletUnlockerClient implements IWalletUnlockerClient, ServiceInfo
      * UnlockWallet is used at startup of lnd to provide a password to unlock
      * the wallet database.
      *
-     * @generated from protobuf rpc: UnlockWallet(lnrpc.UnlockWalletRequest) returns (lnrpc.UnlockWalletResponse);
+     * @generated from protobuf rpc: UnlockWallet
      */
     unlockWallet(input: UnlockWalletRequest, options?: RpcOptions): UnaryCall<UnlockWalletRequest, UnlockWalletResponse> {
         const method = this.methods[2], opt = this._transport.mergeOptions(options);
@@ -171,7 +171,7 @@ export class WalletUnlockerClient implements IWalletUnlockerClient, ServiceInfo
      * ChangePassword changes the password of the encrypted wallet. This will
      * automatically unlock the wallet database if successful.
      *
-     * @generated from protobuf rpc: ChangePassword(lnrpc.ChangePasswordRequest) returns (lnrpc.ChangePasswordResponse);
+     * @generated from protobuf rpc: ChangePassword
      */
     changePassword(input: ChangePasswordRequest, options?: RpcOptions): UnaryCall<ChangePasswordRequest, ChangePasswordResponse> {
         const method = this.methods[3], opt = this._transport.mergeOptions(options);