Add schema version constant and checks

docs/adr/DP-001_schema_v1.1.md

@@ -36,6 +36,7 @@ All foreign keys use `ON DELETE CASCADE` so deleting a file, tag, etc. automatic
    3. **0003\_create\_links\_collections\_views.sql** – introduce `links`, `collections`, `collection_files`, and `views` tables.
    4. **0004\_fix\_hierarchical\_tags\_fts.sql** – refine FTS triggers to index full hierarchical tag-paths via a recursive CTE.
 3. Expose this schema through our library (`libmarlin::db::open`) so any client sees a v1.1 store.
+4. Track the version in code via `SCHEMA_VERSION` and provide `current_schema_version()` to query the DB.
 
 ## 3. ER Diagram
 

libmarlin/src/db/mod.rs

@@ -18,6 +18,11 @@ use rusqlite::{
 use std::result::Result as StdResult;
 use tracing::{debug, info, warn};
 
+/* ─── schema version ───────────────────────────────────────────────── */
+
+/// Current library schema version.
+pub const SCHEMA_VERSION: i32 = 1_1;
+
 /* ─── embedded migrations ─────────────────────────────────────────── */
 
 const MIGRATIONS: &[(&str, &str)] = &[
@@ -47,6 +52,18 @@ const MIGRATIONS: &[(&str, &str)] = &[
     ),
 ];
 
+/* ─── schema helpers ─────────────────────────────────────────────── */
+
+/// Fetch the highest version recorded in the `schema_version` table.
+pub fn current_schema_version(conn: &Connection) -> Result<i32> {
+    let version: i32 = conn.query_row(
+        "SELECT IFNULL(MAX(version), 0) FROM schema_version",
+        [],
+        |r| r.get(0),
+    )?;
+    Ok(version)
+}
+
 /* ─── connection bootstrap ────────────────────────────────────────── */
 
 pub fn open<P: AsRef<Path>>(db_path: P) -> Result<Connection> {
@@ -133,6 +150,15 @@ pub(crate) fn apply_migrations(conn: &mut Connection) -> Result<()> {
         warn!("migrations not applied: {:?}", missing);
     }
 
+    let current = current_schema_version(conn)?;
+    if current != SCHEMA_VERSION {
+        anyhow::bail!(
+            "database schema version {} does not match library version {}",
+            current,
+            SCHEMA_VERSION
+        );
+    }
+
     Ok(())
 }