Fix duplicate migration version numbers (000048/000049)

Renumber migrations from merged feature/ai-feature-activation branch
to resolve golang-migrate duplicate version conflict:
- 000048_add_captain_auto_reply_rules → 000050
- 000049_add_article_embedding_vector → 000051

Both migrations were already applied to the database; schema_migrations
version updated from 49 to 51 to reflect the new numbering.

backend/migrations/000050_add_captain_auto_reply_rules.up.sql

@@ -0,0 +1,30 @@
+-- 000050_add_captain_auto_reply_rules.up.sql
+-- Captain AI — Auto-Reply Rules table
+-- Reference: M12 PRD §Captain AI — Auto-Reply Rules
+-- Stores rules for automated replies when incoming messages match conditions.
+
+CREATE TABLE IF NOT EXISTS captain_auto_reply_rules (
+    id                  BIGSERIAL PRIMARY KEY,
+    account_id          BIGINT       NOT NULL,
+    assistant_id        BIGINT       NOT NULL,
+    inbox_id            BIGINT       NULL,
+    name                VARCHAR(255) NOT NULL,
+    description         TEXT         DEFAULT '',
+    status              VARCHAR(50)  NOT NULL DEFAULT 'draft',
+    mode                VARCHAR(50)  NOT NULL DEFAULT 'static',
+    priority            INTEGER      NOT NULL DEFAULT 0,
+    conditions          JSONB        NOT NULL DEFAULT '[]'::jsonb,
+    response_text       TEXT         DEFAULT '',
+    llm_prompt_override TEXT         DEFAULT '',
+    delay_seconds       INTEGER      NOT NULL DEFAULT 0,
+    one_time_only       BOOLEAN      NOT NULL DEFAULT TRUE,
+    created_at          TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at          TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    deleted_at          TIMESTAMP WITH TIME ZONE NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_captain_auto_reply_rules_account_id ON captain_auto_reply_rules(account_id);
+CREATE INDEX IF NOT EXISTS idx_captain_auto_reply_rules_assistant_id ON captain_auto_reply_rules(assistant_id);
+CREATE INDEX IF NOT EXISTS idx_captain_auto_reply_rules_inbox_id ON captain_auto_reply_rules(inbox_id);
+CREATE INDEX IF NOT EXISTS idx_captain_auto_reply_rules_status ON captain_auto_reply_rules(status);
+CREATE INDEX IF NOT EXISTS idx_captain_auto_reply_rules_deleted_at ON captain_auto_reply_rules(deleted_at);