feat(media): multimodal attachments — image/video inlining per model capabilities

Adds media.rs for per-turn attachment routing, message_builder
partitioning by resolved model capabilities, controller endpoints
for uploads, and server routing for /data/* behind session auth.

See CLAUDE.md §Multimodal attachments for full design.

crates/skald-core/src/llm/mod.rs

@@ -23,6 +23,9 @@ pub struct LlmEntry {
     pub context_length:  Option<i64>,
     /// When true, prompt-caching hints are injected into requests.
     pub prompt_cache:    bool,
+    /// Input capabilities of the resolved model (`vision`, `video`, …), from
+    /// `llm_models.capabilities`. Drives multimodal attachment inlining.
+    pub capabilities:    Vec<String>,
 }
 
 // ── Provider ──────────────────────────────────────────────────────────────────