perf: Configurable rate limit, 429 auto-retry, concurrent project ingestion

The sync pipeline was bottlenecked at 10 req/s (hardcoded) with
sequential project processing and no retry on rate limiting. These
changes target 3-5x throughput improvement.

Rate limit configuration:
- Add requestsPerSecond to SyncConfig (default 30.0, was hardcoded 10)
- Pass configured rate through to GitLabClient::new from ingest
- Floor rate at 0.1 rps in RateLimiter::new to prevent panic on
  Duration::from_secs_f64(1.0 / 0.0) — now reachable via user config

429 auto-retry:
- Both request() and request_with_headers() retry up to 3 times on
  HTTP 429, respecting the retry-after header (default 60s)
- Extract parse_retry_after helper, reused by handle_response fallback
- After exhausting retries, the 429 error propagates as before
- Improved JSON decode errors now include a response body preview

Concurrent project ingestion:
- Derive Clone on GitLabClient (cheap: shares Arc<Mutex<RateLimiter>>
  and reqwest::Client which is already Arc-backed)
- Restructure project loop to use futures::stream::buffer_unordered
  with primary_concurrency (default 4) as the parallelism bound
- Each project gets its own SQLite connection (WAL mode + busy_timeout
  handles concurrent writes)
- Add show_spinner field to IngestDisplay to separate the per-project
  spinner from the sync-level stage spinner
- Error aggregation defers failures: all successful projects get their
  summaries printed and results counted before returning the first error
- Bump dependentConcurrency default from 2 to 8 for discussion prefetch

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/config.rs

@@ -51,6 +51,9 @@ pub struct SyncConfig {
     #[serde(rename = "dependentConcurrency")]
     pub dependent_concurrency: u32,
 
+    #[serde(rename = "requestsPerSecond")]
+    pub requests_per_second: f64,
+
     #[serde(rename = "fetchResourceEvents", default = "default_true")]
     pub fetch_resource_events: bool,
 }
@@ -67,7 +70,8 @@ impl Default for SyncConfig {
             heartbeat_interval_seconds: 30,
             cursor_rewind_seconds: 2,
             primary_concurrency: 4,
-            dependent_concurrency: 2,
+            dependent_concurrency: 8,
+            requests_per_second: 30.0,
             fetch_resource_events: true,
         }
     }