feat: unified provider resolver with protocol-based dispatch

Extract provider resolution logic into two new modules:
- provider-catalog.ts: vendor presets, default model catalogs, protocol definitions
- provider-resolver.ts: unified resolution for all consumers (Claude SDK, AI SDK, model listing)

DB migration:
- Add protocol, headers_json, env_overrides_json, role_models_json columns to api_providers
- Create provider_models table for per-provider model catalog

Consumer refactoring:
- claude-client.ts: replace inline env-building with toClaudeCodeEnv()
- text-generator.ts: replace inline config with toAiSdkConfig()
- conversation-engine.ts, onboarding, checkin, plan routes: use shared resolveProvider()
- /api/providers/models: read from catalog + provider_models table

UI improvements:
- Merge confusing "Custom API (Anthropic-compatible)" into "Anthropic Third-party API"
- Add 3-field model name mapping (sonnet/opus/haiku) in advanced options
- ProviderForm.tsx: support new fields (protocol, env_overrides, role_models)

Bug fixes:
- Protect auth credentials from being deleted by legacy extra_env empty-string overrides
- Fix env var names: ANTHROPIC_DEFAULT_SONNET → ANTHROPIC_DEFAULT_SONNET_MODEL (SDK convention)

Tests: 229/229 passed. Verified with aiberm (no mapping) and pipellm (with mapping).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -32,8 +32,10 @@
     ]
   },
   "dependencies": {
+    "@ai-sdk/amazon-bedrock": "^4.0.77",
     "@ai-sdk/anthropic": "^3.0.47",
     "@ai-sdk/google": "^3.0.31",
+    "@ai-sdk/google-vertex": "^4.0.80",
     "@ai-sdk/openai": "^3.0.34",
     "@anthropic-ai/claude-agent-sdk": "^0.2.62",
     "@google/genai": "^1.43.0",