fix(#275): Prevent silent connection initiation failures

Fixed silent connection initiation failures where HTTP errors were caught
but success was returned to the user, leaving zombie connections in
PENDING state forever.

Changes:
- Delete failed connection from database when HTTP request fails
- Throw BadRequestException with clear error message
- Added test to verify connection deletion and exception throwing
- Import BadRequestException in connection.service.ts

User Impact:
- Users now receive immediate feedback when connection initiation fails
- No more zombie connections stuck in PENDING state
- Clear error messages indicate the reason for failure

Testing:
- Added test case: "should delete connection and throw error if request fails"
- All 21 connection service tests passing
- Quality gates: lint, typecheck, build all passing

Fixes #275

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/scratchpads/275-silent-connection-failures.md

@@ -0,0 +1,82 @@
+# Issue #275: Fix silent connection initiation failures
+
+## Objective
+
+Fix silent connection initiation failures where HTTP errors are caught but success is returned to the user, leaving zombie connections in PENDING state forever.
+
+## Location
+
+`apps/api/src/federation/connection.service.ts:72-80`
+
+## Problem
+
+Current code:
+
+```typescript
+try {
+  await firstValueFrom(
+    this.httpService.post(`${remoteUrl}/api/v1/federation/incoming/connect`, signedRequest)
+  );
+  this.logger.log(`Connection request sent to ${remoteUrl}`);
+} catch (error) {
+  this.logger.error(`Failed to send connection request to ${remoteUrl}`, error);
+  // Connection is still created in PENDING state, can be retried
+}
+
+return this.mapToConnectionDetails(connection);
+```
+
+Issues:
+
+- Catches HTTP failures but returns success
+- Connection stays in PENDING state forever
+- Creates zombie connections
+- User sees success message but connection actually failed
+
+## Solution
+
+1. Delete the failed connection from database
+2. Throw exception with clear error message
+3. User gets immediate feedback that connection failed
+
+## Implementation
+
+```typescript
+try {
+  await firstValueFrom(
+    this.httpService.post(`${remoteUrl}/api/v1/federation/incoming/connect`, signedRequest)
+  );
+  this.logger.log(`Connection request sent to ${remoteUrl}`);
+} catch (error) {
+  this.logger.error(`Failed to send connection request to ${remoteUrl}`, error);
+
+  // Delete the failed connection to prevent zombie connections
+  await this.prisma.federationConnection.delete({
+    where: { id: connection.id },
+  });
+
+  throw new BadRequestException(
+    `Failed to initiate connection to ${remoteUrl}: ${error instanceof Error ? error.message : "Unknown error"}`
+  );
+}
+```
+
+## Testing
+
+Test scenarios:
+
+1. Remote instance is unreachable - should throw exception and delete connection
+2. Remote instance returns error - should throw exception and delete connection
+3. Remote instance times out - should throw exception and delete connection
+4. Remote instance returns success - should create connection in PENDING state
+
+## Progress
+
+- [ ] Create scratchpad
+- [ ] Implement fix in connection.service.ts
+- [ ] Add/update tests
+- [ ] Run quality gates
+- [ ] Commit changes
+- [ ] Create PR
+- [ ] Merge to develop
+- [ ] Close issue #275