feat(ai): thinking mode switch, vision image input, and API host normalization fixes

[lb1038678031]

Resubmit of #1594, addressing @LinkinStars's feedback there. Implements #1595 (feature clarification).

What

Three improvements to the AI feature, shipped as independent commits:

1. fix(ai): normalize API host and harden AI model listing

   • The models endpoint used to call api_host + "/v1/models" while the chat
     client auto-appends /v1. Configuring a base URL that already contains /v1
     (common with OpenAI-compatible gateways such as SenseNova, SiliconFlow,
     OneAPI relays...) produced /v1/v1/models, and the gateway's gRPC-style
     NOT_FOUND error was passed through to the UI.
   • New shared helper schema.NormalizeAPIHost() trims slashes/whitespace,
     appends /v1 when missing, and keeps /v1beta/* endpoints intact.
   • Upstream error bodies are summarized (error.code/message) instead of being
     echoed verbatim; image data is never logged.
   • Stored-key lookup prefers the chosen provider before falling back to host
     matching.
   • Default provider list: Gemini points at its OpenAI-compatible endpoint;
     Anthropic removed because the backend only speaks the OpenAI protocol.

2. feat(ai): per-provider thinking mode

   • New thinking_mode switch on each provider in Admin → AI settings.
   • When enabled, a custom http.RoundTripper merges "enable_thinking": true
     into chat completion request bodies — the convention used by
     reasoning-capable OpenAI-compatible gateways (DeepSeek V4, Qwen/DashScope,
     SenseNova, vLLM/SGLang, ...). The existing reasoning_content streaming
     path renders the thought process in the bubble UI unchanged.

3. feat(ai): vision image input behind an admin switch

   • New vision_enabled switch per provider; exposed to clients via site info
     as ai_vision_enabled.
   • Users can attach up to 4 PNG/JPEG/WebP images (≤4 MB decoded each) to the
     first message of a turn; images travel as base64 data URLs or HTTPS links
     and are converted into MultiContent parts.
   • Conversation history stores a textual placeholder instead of raw image data
     (no DB schema change).

Security note (from #1594)

The base64 constant flagged during review was the encoding of a standard 1×1
pixel PNG placeholder used in a unit test, not a credential. To avoid any
misreading, 158516b constructs the placeholder at runtime from the canonical
PNG signature bytes; the source no longer contains any base64 blob.

Testing

• go test ./internal/schema/ ./internal/controller/ ./internal/service/siteinfo/ ./internal/migrations/
  — new table tests for host normalization, transport injection, and image
  validation (count/type/size).
• Manual smoke against a SenseNova OpenAI-compatible endpoint: model listing
  succeeds with both root and /v1 base URLs; reasoning_content appears with
  thinking enabled; image questions answered by a vision-capable model.