Refuse an exclude pattern that does not compile at config load (closes #455)

An exclude pattern that did not compile was skipped with a warning during the backup, which then stored what the user meant to leave out. Config.Validate now refuses it, naming the setting and quoting the pattern, for example exclude[2]: "build[" is not a valid exclude pattern: ...

The compile step moves to a new package, internal/exclude, as exclude.Compile; Validate and the scanner both call it. The scanner's skip-and-warn path is gone; a pattern that does not compile there now fails the scanner.

Model: opus-5-5

internal/config/config.go

@@ -396,6 +396,8 @@ func Parse(data []byte) (*Config, error) {
 //     never the value. An empty list is accepted, because only snapshot
 //     create needs a recipient and it checks for one itself
 //   - At least one snapshot must be configured with at least one path
+//   - Every exclude pattern, global or in a snapshot, must compile; errors
+//     name the setting and quote the pattern
 //   - Storage must be configured (either storage_url or s3.* fields)
 //   - The index path must be absolute: a relative one would resolve against
 //     the working directory, so each directory would get its own index
@@ -416,18 +418,18 @@ func (c *Config) Validate() error {
 		}
 	}
 
-	if len(c.Snapshots) == 0 {
-		return errNoSnapshots
+	err := c.validateSnapshots()
+	if err != nil {
+		return err
 	}
 
-	for name, snap := range c.Snapshots {
-		if len(snap.Paths) == 0 {
-			return fmt.Errorf("%w: %q", errSnapshotNoPaths, name)
-		}
+	err = c.validateExcludes()
+	if err != nil {
+		return err
 	}
 
 	// Validate storage configuration
-	err := c.validateStorage()
+	err = c.validateStorage()
 	if err != nil {
 		return err
 	}
@@ -466,6 +468,22 @@ func (c *Config) Validate() error {
 	return nil
 }
 
+// validateSnapshots refuses a config with no snapshot or with a snapshot
+// that has no path.
+func (c *Config) validateSnapshots() error {
+	if len(c.Snapshots) == 0 {
+		return errNoSnapshots
+	}
+
+	for name, snap := range c.Snapshots {
+		if len(snap.Paths) == 0 {
+			return fmt.Errorf("%w: %q", errSnapshotNoPaths, name)
+		}
+	}
+
+	return nil
+}
+
 // validateAgeRecipient parses one age_recipients entry with the age library
 // and returns a value-free error on failure. A recipient string can be
 // sensitive (an operator may paste a secret key by mistake), so neither the