Phase 4: policy engine with deny-wins, default-deny evaluation

internal/policy/:
Priority-based policy engine per ARCHITECTURE.md §4. Stateless
Evaluate() sorts rules by priority, collects all matches, deny-wins
over allow, default-deny if no match. Rule matching: all populated
fields ANDed, empty fields are wildcards, repository glob via
path.Match. Built-in defaults: admin wildcard (all actions), human
user content access (pull/push/delete/catalog), version check
(always accessible). Engine wrapper with sync.RWMutex-protected
cache, SetRules merges with defaults, Reload loads from RuleStore.

internal/db/:
LoadEnabledPolicyRules() parses rule_json column from policy_rules
table into []policy.Rule, filtered by enabled=1, ordered by priority.

internal/server/:
RequirePolicy middleware extracts claims from context, repo from chi
URL param, evaluates policy, returns OCI DENIED (403) on deny with
optional audit callback.

69 tests passing across all packages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/db/policy.go

@@ -0,0 +1,68 @@
+package db
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"git.wntrmute.dev/kyle/mcr/internal/policy"
+)
+
+// ruleBody is the JSON structure stored in the rule_json column.
+type ruleBody struct {
+	Effect       string   `json:"effect"`
+	Roles        []string `json:"roles,omitempty"`
+	AccountTypes []string `json:"account_types,omitempty"`
+	SubjectUUID  string   `json:"subject_uuid,omitempty"`
+	Actions      []string `json:"actions"`
+	Repositories []string `json:"repositories,omitempty"`
+}
+
+// LoadEnabledPolicyRules returns all enabled policy rules from the database,
+// ordered by priority ascending. It implements policy.RuleStore.
+func (d *DB) LoadEnabledPolicyRules() ([]policy.Rule, error) {
+	rows, err := d.Query(
+		`SELECT id, priority, description, rule_json
+		 FROM policy_rules
+		 WHERE enabled = 1
+		 ORDER BY priority ASC`,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("db: load policy rules: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	var rules []policy.Rule
+	for rows.Next() {
+		var id int64
+		var priority int
+		var description, ruleJSON string
+		if err := rows.Scan(&id, &priority, &description, &ruleJSON); err != nil {
+			return nil, fmt.Errorf("db: scan policy rule: %w", err)
+		}
+
+		var body ruleBody
+		if err := json.Unmarshal([]byte(ruleJSON), &body); err != nil {
+			return nil, fmt.Errorf("db: parse rule_json for rule %d: %w", id, err)
+		}
+
+		rule := policy.Rule{
+			ID:           id,
+			Priority:     priority,
+			Description:  description,
+			Effect:       policy.Effect(body.Effect),
+			Roles:        body.Roles,
+			AccountTypes: body.AccountTypes,
+			SubjectUUID:  body.SubjectUUID,
+			Repositories: body.Repositories,
+		}
+		for _, a := range body.Actions {
+			rule.Actions = append(rule.Actions, policy.Action(a))
+		}
+		rules = append(rules, rule)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("db: iterate policy rules: %w", err)
+	}
+
+	return rules, nil
+}