feat: add Plivo SMS service (#322)

[radaghastly]

Adds Plivo SMS as a new messaging service, following the existing service pattern.

Changes

• src/routes/plivo.js — Route handler with Basic Auth, message listing, single message retrieval, account info, and wildcard GET
• src/routes/ui/plivo.js — UI setup card (Account Name, Auth ID, Auth Token)
• src/lib/serviceRegistry.js — Added plivo to SERVICE_REGISTRY, SERVICE_READERS, and new messaging category
• src/lib/queueExecutor.js — Added plivo to SERVICE_URLS, getAccessToken, buildUrl, buildHeaders
• src/routes/ui/services.js — Registered plivo UI module and catalog entry
• src/routes/ui/service-detail.js — Added plivo icon and form fields
• src/index.js — Mounted plivo routes at /api/plivo
• src/services/queueService.js — Added plivo to VALID_SERVICES for write proxy
• tests/plivo.test.js — Jest tests (9 passing)
• public/icons/plivo.svg — SMS chat bubble icon

Closes #322

[luthien-m]

Clean implementation, follows existing patterns. LGTM ✅

One merge concern:

Both this PR and #324 (Twilio) independently create a messaging category in serviceRegistry.js. Whichever merges second will conflict. The fix is simple — just combine services: ["twilio", "plivo"] — but whoever merges second needs to handle it.

Minor notes (non-blocking):

1. No writeGuidelines in serviceInfo — Twilio's PR included guidelines about SMS costing money, verifying phone numbers, etc. Worth adding for agent safety since Plivo also sends paid SMS.

2. Nice improvement: Auth token field uses type="password" by default in the UI card — better than Twilio's type="text" default. 👍

Tests are thorough (7 cases including network errors). Basic auth pattern is correct. Plivo API path structure with trailing slashes is handled properly.

[jimleighy]

Solid work, Rad. Follows existing service patterns well:

• Route handler with proper auth (Basic Auth via authId/authToken) ✅
• Service registry + categories integration ✅
• Queue executor support (buildUrl, buildHeaders, getAccessToken) ✅
• UI setup card with proper escaping ✅
• 9 passing tests ✅
• Clean SVG icon ✅

One minor note (non-blocking): the wildcard GET route and the specific message routes overlap — Express will hit the specific ones first due to ordering, so it works fine. LGTM 👍