ADR-006 v2.0: Sidecar V2 architecture revision based on review feedback

Incorporated feedback from 4 reviewers:
- 徐聪: AES key management, emergency channel, concurrency control, DDL indexes
- 陆怀瑾: P0 phase, schedule buffer, deployment topology, V1 compat checklist
- 严维序: SQLite backup, monitoring, cooldown persistence, port plan, rollback
- 沈路明: queue design, health check, per-model RPM decision, key validation, dashboard panels

Key additions:
+ Queue flow control design (FIFO + priority, capacity 500, REJECT overflow)
+ Provider health check (active probe + passive stats hybrid)
+ Per-model RPM decision (Provider-level V2, Model-level V3)
+ Key validation on add (test call with error feedback)
+ AES key management (SIDECAR_ENCRYPTION_KEY env var, backup SOP)
+ Emergency channel (10% RPM during full cooldown)
+ SQLite backup strategy (cron .backup, 7-day retention)
+ SQLite monitoring Prometheus metrics (db_size, wal_size, integrity)
+ Full DDL with indexes (ON CONFLICT, BEGIN IMMEDIATE patterns)
+ Dashboard panel list (5 panels: status, trends, history)
+ V1 compatibility checklist (13 items)
+ V1->V2 migration SOP with rollback plan
+ Deployment topology (systemd + Docker, port plan, firewall)
+ Log aggregation policy (logrotate: 10MB/30days)
+ Schedule revised: 71h/12days (added P0 + buffer)

Co-authored-by: multica-agent <github@multica.ai>

docs/architecture/sidecar-v2-er.mmd

@@ -0,0 +1,71 @@
+erDiagram
+    providers ||--o{ provider_usage_logs : has
+    providers ||--o{ cooldown_events : triggers
+    providers ||--o| provider_health : monitors
+
+    providers {
+        string id PK
+        string name
+        string api_key
+        string endpoint_url
+        string model_prefix
+        string pool
+        string status
+        string source
+        int rpm_limit
+        int tpm_limit
+        float weight
+        float cost_per_1k
+        string cooldown_until
+        string metadata
+    }
+
+    provider_usage_logs {
+        string id PK
+        string provider_id FK
+        string model
+        int prompt_tokens
+        int completion_tokens
+        int total_tokens
+        float cost
+        int request_count
+        int error_count
+        int avg_latency_ms
+        string hour_bucket
+    }
+
+    cooldown_events {
+        string id PK
+        string provider_id FK
+        int consecutive_count
+        int cooldown_seconds
+        string response_summary
+        string started_at
+        string ended_at
+    }
+
+    provider_health {
+        string provider_id PK
+        string state
+        int last_latency_ms
+        int last_status_code
+        float success_rate_5m
+        int consecutive_failures
+    }
+
+    daily_stats {
+        string id PK
+        string date
+        string pool
+        int total_requests
+        int total_errors
+        int total_tokens
+        float total_cost
+        int unique_providers
+    }
+
+    system_config {
+        string key PK
+        string value
+        string description
+    }