ai: send images directly to llamacpp chat models + add ResolvedBackend

llamacpp models now receive images via OpenAI content-parts instead of
the describe-then-inline strategy (hybrid mode unchanged). Fixes
assistant messages with tool_calls emitting content: null instead of ""
to satisfy strict Jinja template role-alternation checks. Adds debug
logging of message role sequences on llamacpp requests.

Introduces BackendKind enum, SamplingOverrides, and ResolvedBackend in
a new backend.rs module. InsightGenerator::resolve_backend centralises
client construction + vision capability detection — next step wires the
existing inline dispatch through it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/backend.rs

@@ -0,0 +1,118 @@
+use anyhow::{Result, anyhow};
+
+use crate::ai::llm_client::LlmClient;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BackendKind {
+    Local,
+    Hybrid,
+}
+
+impl BackendKind {
+    pub fn parse(s: &str) -> Result<Self> {
+        match s.trim().to_lowercase().as_str() {
+            "local" | "" => Ok(Self::Local),
+            "hybrid" => Ok(Self::Hybrid),
+            other => Err(anyhow!("unknown backend '{}'; expected 'local' or 'hybrid'", other)),
+        }
+    }
+
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::Local => "local",
+            Self::Hybrid => "hybrid",
+        }
+    }
+}
+
+impl std::fmt::Display for BackendKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+pub struct SamplingOverrides {
+    pub model: Option<String>,
+    pub num_ctx: Option<i32>,
+    pub temperature: Option<f32>,
+    pub top_p: Option<f32>,
+    pub top_k: Option<i32>,
+    pub min_p: Option<f32>,
+}
+
+impl SamplingOverrides {
+    pub fn has_sampling(&self) -> bool {
+        self.temperature.is_some()
+            || self.top_p.is_some()
+            || self.top_k.is_some()
+            || self.min_p.is_some()
+    }
+}
+
+pub struct ResolvedBackend {
+    chat: Box<dyn LlmClient>,
+    local: Box<dyn LlmClient>,
+    pub kind: BackendKind,
+    /// `true` when the chat model receives images directly (Ollama with
+    /// vision, or llamacpp). `false` for hybrid where we describe-then-inline.
+    pub images_inline: bool,
+}
+
+impl ResolvedBackend {
+    pub fn new(
+        chat: Box<dyn LlmClient>,
+        local: Box<dyn LlmClient>,
+        kind: BackendKind,
+        images_inline: bool,
+    ) -> Self {
+        Self { chat, local, kind, images_inline }
+    }
+
+    pub fn chat(&self) -> &dyn LlmClient {
+        self.chat.as_ref()
+    }
+
+    pub fn local(&self) -> &dyn LlmClient {
+        self.local.as_ref()
+    }
+
+    pub fn model(&self) -> &str {
+        self.chat.primary_model()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_backend_kind() {
+        assert_eq!(BackendKind::parse("local").unwrap(), BackendKind::Local);
+        assert_eq!(BackendKind::parse("hybrid").unwrap(), BackendKind::Hybrid);
+        assert_eq!(BackendKind::parse("  Local ").unwrap(), BackendKind::Local);
+        assert_eq!(BackendKind::parse("HYBRID").unwrap(), BackendKind::Hybrid);
+        assert_eq!(BackendKind::parse("").unwrap(), BackendKind::Local);
+        assert!(BackendKind::parse("vllm").is_err());
+    }
+
+    #[test]
+    fn backend_kind_as_str_roundtrips() {
+        assert_eq!(BackendKind::parse(BackendKind::Local.as_str()).unwrap(), BackendKind::Local);
+        assert_eq!(BackendKind::parse(BackendKind::Hybrid.as_str()).unwrap(), BackendKind::Hybrid);
+    }
+
+    #[test]
+    fn sampling_overrides_has_sampling() {
+        let empty = SamplingOverrides {
+            model: None, num_ctx: None, temperature: None,
+            top_p: None, top_k: None, min_p: None,
+        };
+        assert!(!empty.has_sampling());
+
+        let with_temp = SamplingOverrides {
+            model: None, num_ctx: Some(4096), temperature: Some(0.7),
+            top_p: None, top_k: None, min_p: None,
+        };
+        assert!(with_temp.has_sampling());
+    }
+}