feat: add issue_usage tracking and metrics for AI ticket enhancement and planning

Add `issue_usage` table with columns for user_uid, tokens, cost, and latency metrics, including a unique index on user_uid. Wire `accumulate_usage` into `enhance_ticket` and `generate_plan` to capture per-request AI usage, persist totals via `add_issue_usage` in both `IssueCreateService` and `PlanningReportService`, and expose aggregated usage as metric cards through `IssueTrackerService.collect_metrics`. Update planning API docs summary to reflect the new phased implementation document format.

tests/unit/database.py

@@ -222,6 +222,43 @@ def test_add_news_usage_ignores_zero_call_batches(local_db):
     assert get_news_usage()["calls"] == 0
 
 
+def test_issue_usage_accumulates_and_computes_averages(local_db):
+    from devplacepy.database import add_issue_usage, get_issue_usage
+
+    get_table("issue_usage").delete()
+    empty = get_issue_usage()
+    assert empty["calls"] == 0
+    assert empty["avg_tokens"] == 0.0
+
+    totals = {
+        "calls": 2,
+        "prompt_tokens": 2000,
+        "completion_tokens": 400,
+        "total_tokens": 2400,
+        "cost_usd": 0.0005,
+        "upstream_latency_ms": 1600.0,
+        "total_latency_ms": 1800.0,
+    }
+    add_issue_usage(totals)
+    add_issue_usage(totals)
+
+    usage = get_issue_usage()
+    assert usage["calls"] == 4
+    assert usage["total_tokens"] == 4800
+    assert abs(usage["cost_usd"] - 0.001) < 1e-9
+    assert usage["avg_tokens"] == 1200.0
+    assert usage["avg_cost_usd"] == 0.00025
+    assert usage["avg_tokens_per_second"] == 250.0
+
+
+def test_add_issue_usage_ignores_zero_call_batches(local_db):
+    from devplacepy.database import add_issue_usage, get_issue_usage
+
+    get_table("issue_usage").delete()
+    add_issue_usage({"calls": 0, "total_tokens": 999, "cost_usd": 1.0})
+    assert get_issue_usage()["calls"] == 0
+
+
 def test_interleave_by_author_spreads_consecutive_runs():
     from devplacepy.database import interleave_by_author