feat(11-02): add findByNumber and getNextNumber to PhaseRepository

- Add findByNumber method to lookup phase by initiative and number
- Add getNextNumber method to get next available phase number
- Implement both methods in DrizzlePhaseRepository adapter
- Use drizzle-orm max() and and() for query construction

src/db/repositories/drizzle/phase.ts

@@ -4,7 +4,7 @@
  * Implements PhaseRepository interface using Drizzle ORM.
  */
 
-import { eq, asc } from 'drizzle-orm';
+import { eq, asc, and, max } from 'drizzle-orm';
 import { nanoid } from 'nanoid';
 import type { DrizzleDatabase } from '../../index.js';
 import { phases, type Phase } from '../../schema.js';
@@ -58,6 +58,26 @@ export class DrizzlePhaseRepository implements PhaseRepository {
       .orderBy(asc(phases.number));
   }
 
+  async findByNumber(initiativeId: string, number: number): Promise<Phase | null> {
+    const result = await this.db
+      .select()
+      .from(phases)
+      .where(and(eq(phases.initiativeId, initiativeId), eq(phases.number, number)))
+      .limit(1);
+
+    return result[0] ?? null;
+  }
+
+  async getNextNumber(initiativeId: string): Promise<number> {
+    const result = await this.db
+      .select({ maxNumber: max(phases.number) })
+      .from(phases)
+      .where(eq(phases.initiativeId, initiativeId));
+
+    const maxNumber = result[0]?.maxNumber ?? 0;
+    return maxNumber + 1;
+  }
+
   async update(id: string, data: UpdatePhaseData): Promise<Phase> {
     const existing = await this.findById(id);
     if (!existing) {

src/db/repositories/phase-repository.ts

@@ -46,6 +46,18 @@ export interface PhaseRepository {
    */
   findByInitiativeId(initiativeId: string): Promise<Phase[]>;
 
+  /**
+   * Find a phase by initiative and number.
+   * Returns null if not found.
+   */
+  findByNumber(initiativeId: string, number: number): Promise<Phase | null>;
+
+  /**
+   * Get the next available phase number for an initiative.
+   * Returns MAX(number) + 1, or 1 if no phases exist.
+   */
+  getNextNumber(initiativeId: string): Promise<number>;
+
   /**
    * Update a phase.
    * Throws if phase not found.