feat: create llm_call_log table + DB module

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

CLAUDE.md

@@ -117,7 +117,7 @@ cd frontend && npx tsc --noEmit
 - `GET /api/v1/admin/users` — user list
 - `PUT /api/v1/admin/users/:id/role` — role management
 
-## Database (16 migrations)
+## Database (17 migrations)
 Tables: `users`, `sessions`, `magic_link_tokens`, `user_settings`, `sources`, `syntheses`, `admin_providers`, `admin_rate_limits`, `user_api_keys`, `audit_log`
 
 ## Environment Variables

backend/migrations/20260324000017_create_llm_call_log.sql

@@ -0,0 +1,14 @@
+CREATE TABLE llm_call_log (
+    id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id         UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    job_id          UUID NOT NULL,
+    call_type       TEXT NOT NULL,
+    model           TEXT NOT NULL,
+    system_prompt   TEXT NOT NULL DEFAULT '',
+    user_prompt     TEXT NOT NULL DEFAULT '',
+    response_body   TEXT NOT NULL DEFAULT '',
+    duration_ms     INTEGER NOT NULL DEFAULT 0,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+CREATE INDEX idx_llm_call_log_job_id ON llm_call_log(job_id);
+CREATE INDEX idx_llm_call_log_user_id ON llm_call_log(user_id, created_at);

backend/src/db/llm_call_log.rs

@@ -0,0 +1,97 @@
+//! LLM call logging: tracks every LLM interaction during synthesis generation.
+
+use chrono::{DateTime, Utc};
+use serde::Serialize;
+use sqlx::PgPool;
+use uuid::Uuid;
+
+use crate::errors::AppError;
+
+/// Row returned from llm_call_log queries.
+#[derive(Debug, Clone, Serialize, sqlx::FromRow)]
+pub struct LlmCallLogRow {
+    pub id: Uuid,
+    pub call_type: String,
+    pub model: String,
+    pub system_prompt: String,
+    pub user_prompt: String,
+    pub response_body: String,
+    pub duration_ms: i32,
+    pub created_at: DateTime<Utc>,
+}
+
+/// Insert a single LLM call log entry.
+pub async fn insert(
+    pool: &PgPool,
+    user_id: Uuid,
+    job_id: Uuid,
+    call_type: &str,
+    model: &str,
+    system_prompt: &str,
+    user_prompt: &str,
+    response_body: &str,
+    duration_ms: i32,
+) -> Result<(), AppError> {
+    sqlx::query(
+        r#"
+        INSERT INTO llm_call_log (user_id, job_id, call_type, model, system_prompt, user_prompt, response_body, duration_ms)
+        VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+        "#,
+    )
+    .bind(user_id)
+    .bind(job_id)
+    .bind(call_type)
+    .bind(model)
+    .bind(system_prompt)
+    .bind(user_prompt)
+    .bind(response_body)
+    .bind(duration_ms)
+    .execute(pool)
+    .await?;
+    Ok(())
+}
+
+/// List all LLM call log entries for a generation job.
+pub async fn list_by_job_id(
+    pool: &PgPool,
+    user_id: Uuid,
+    job_id: Uuid,
+) -> Result<Vec<LlmCallLogRow>, AppError> {
+    let rows = sqlx::query_as::<_, LlmCallLogRow>(
+        r#"
+        SELECT id, call_type, model, system_prompt, user_prompt, response_body, duration_ms, created_at
+        FROM llm_call_log
+        WHERE user_id = $1 AND job_id = $2
+        ORDER BY created_at ASC
+        "#,
+    )
+    .bind(user_id)
+    .bind(job_id)
+    .fetch_all(pool)
+    .await?;
+    Ok(rows)
+}
+
+/// Truncate old LLM call log entries: replace prompt/response with first 500 chars.
+pub async fn truncate_old(
+    pool: &PgPool,
+    user_id: Uuid,
+    days: i32,
+) -> Result<u64, AppError> {
+    let result = sqlx::query(
+        r#"
+        UPDATE llm_call_log
+        SET system_prompt = LEFT(system_prompt, 500) || E'\n[truncated]',
+            user_prompt = LEFT(user_prompt, 500) || E'\n[truncated]',
+            response_body = LEFT(response_body, 500) || E'\n[truncated]'
+        WHERE user_id = $1
+            AND created_at < now() - make_interval(days => $2)
+            AND LENGTH(system_prompt) > 510
+        "#,
+    )
+    .bind(user_id)
+    .bind(days)
+    .execute(pool)
+    .await?;
+    Ok(result.rows_affected())
+}

backend/src/db/mod.rs

@@ -1,5 +1,6 @@
 pub mod article_history;
 pub mod api_keys;
+pub mod llm_call_log;
 pub mod audit;
 pub mod magic_links;
 pub mod providers;