chore: normalize line endings to LF via .gitattributes

Root-cause fix for mixed CRLF/LF in the repo (no .gitattributes previously).
Adds '* text=auto eol=lf' and explicit binary rules so all future checkouts
are LF. Renormalizes all tracked text files (no logical content change).
Backup tags: image qihuaiflow:backup-lineend-20260725-121632, git backup-lineend-pre-20260725-121632.

migrations/021_create_conversation_summaries.sql

@@ -1,7 +1,7 @@
-CREATE TABLE IF NOT EXISTS conversation_summaries (
-  conversation_id VARCHAR(64) NOT NULL,
-  summary_text TEXT NULL,
-  token_count INT NULL DEFAULT 0,
-  updated_at DATETIME(3) NULL,
-  PRIMARY KEY (conversation_id)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+CREATE TABLE IF NOT EXISTS conversation_summaries (
+  conversation_id VARCHAR(64) NOT NULL,
+  summary_text TEXT NULL,
+  token_count INT NULL DEFAULT 0,
+  updated_at DATETIME(3) NULL,
+  PRIMARY KEY (conversation_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;